diff --git a/lib/Less/Functions.php b/lib/Less/Functions.php
index e958e77c..86595f7e 100644
--- a/lib/Less/Functions.php
+++ b/lib/Less/Functions.php
@@ -697,9 +697,7 @@ private function _minmax( $isMin, $args ) {
 		for ( $i = 0; $i < $arg_count; $i++ ) {
 			$current = $args[$i];
 			if ( !( $current instanceof Less_Tree_Dimension ) ) {
-				// @phan-suppress-next-line PhanUndeclaredProperty Checked Less_Tree->value
-				if ( property_exists( $args[$i], 'value' ) && is_array( $args[$i]->value ) ) {
-					// @phan-suppress-next-line PhanUndeclaredProperty Checked Less_Tree->value
+				if ( $args[$i] instanceof Less_Tree_HasValueProperty && is_array( $args[$i]->value ) ) {
 					$args[] = $args[$i]->value;
 				}
 				continue;
@@ -842,8 +840,7 @@ public function isem( $n ) {
 	 * @param Less_Tree|string $unit
 	 */
 	public function isunit( $n, $unit ) {
-		if ( is_object( $unit ) && property_exists( $unit, 'value' ) ) {
-			// @phan-suppress-next-line PhanUndeclaredProperty Checked Less_Tree->value
+		if ( $unit instanceof Less_Tree_HasValueProperty ) {
 			$unit = $unit->value;
 		}
 
@@ -870,7 +867,7 @@ public function extract( $values, $index ) {
 		$index = (int)$index->value - 1; // (1-based index)
 		// handle non-array values as an array of length 1
 		// return 'undefined' if index is invalid
-		if ( property_exists( $values, 'value' ) && is_array( $values->value ) ) {
+		if ( $values instanceof Less_Tree_HasValueProperty && is_array( $values->value ) ) {
 			if ( isset( $values->value[$index] ) ) {
 				return $values->value[$index];
 			}
@@ -884,7 +881,8 @@ public function extract( $values, $index ) {
 	}
 
 	public function length( $values ) {
-		$n = ( property_exists( $values, 'value' ) && is_array( $values->value ) ) ? count( $values->value ) : 1;
+		$n = ( $values instanceof Less_Tree_HasValueProperty && is_array( $values->value ) ) ?
+			count( $values->value ) : 1;
 		return new Less_Tree_Dimension( $n );
 	}
 
@@ -1005,7 +1003,7 @@ public function svggradient( $direction ) {
 			'<' . $gradientType . 'Gradient id="gradient" gradientUnits="userSpaceOnUse" ' . $gradientDirectionSvg . '>';
 
 		for ( $i = 0; $i < count( $stops ); $i++ ) {
-			if ( is_object( $stops[$i] ) && property_exists( $stops[$i], 'value' ) ) {
+			if ( $stops[$i] instanceof Less_Tree_HasValueProperty ) {
 				$color = $stops[$i]->value[0];
 				$position = $stops[$i]->value[1];
 			} else {
diff --git a/lib/Less/Parser.php b/lib/Less/Parser.php
index e4690f37..f5a34c48 100644
--- a/lib/Less/Parser.php
+++ b/lib/Less/Parser.php
@@ -1508,7 +1508,7 @@ private function parseMixinArgs( $isCall ) {
 
 			if ( $isCall ) {
 				// Variable
-				if ( property_exists( $arg, 'value' ) && count( $arg->value ) == 1 ) {
+				if ( $value instanceof Less_Tree_HasValueProperty && count( $arg->value ) == 1 ) {
 					$val = $arg->value[0];
 				}
 			} else {
diff --git a/lib/Less/Tree.php b/lib/Less/Tree.php
index bbb2bd99..f5ec816d 100644
--- a/lib/Less/Tree.php
+++ b/lib/Less/Tree.php
@@ -2,12 +2,6 @@
 
 /**
  * Tree
- *
- * TODO: Callers often use `property_exists(, 'value')` to distinguish
- * tree nodes that are considerd value-holding. Refactor this to move
- * the 'value' property that most subclasses implement to there, and use
- * something else (special value, method, or intermediate class?) to
- * signal whether a subclass is considered value-holding.
  */
 class Less_Tree {
 
diff --git a/lib/Less/Tree/Alpha.php b/lib/Less/Tree/Alpha.php
index b00bf392..50a7a9a7 100644
--- a/lib/Less/Tree/Alpha.php
+++ b/lib/Less/Tree/Alpha.php
@@ -2,7 +2,7 @@
 /**
  * @private
  */
-class Less_Tree_Alpha extends Less_Tree {
+class Less_Tree_Alpha extends Less_Tree implements Less_Tree_HasValueProperty {
 	public $value;
 
 	public function __construct( $val ) {
diff --git a/lib/Less/Tree/Anonymous.php b/lib/Less/Tree/Anonymous.php
index 20e31b06..84e93a88 100644
--- a/lib/Less/Tree/Anonymous.php
+++ b/lib/Less/Tree/Anonymous.php
@@ -2,7 +2,7 @@
 /**
  * @private
  */
-class Less_Tree_Anonymous extends Less_Tree {
+class Less_Tree_Anonymous extends Less_Tree implements Less_Tree_HasValueProperty {
 	public $value;
 	public $quote;
 	public $index;
diff --git a/lib/Less/Tree/Assignment.php b/lib/Less/Tree/Assignment.php
index 814bd5a3..c17133cf 100644
--- a/lib/Less/Tree/Assignment.php
+++ b/lib/Less/Tree/Assignment.php
@@ -2,7 +2,7 @@
 /**
  * @private
  */
-class Less_Tree_Assignment extends Less_Tree {
+class Less_Tree_Assignment extends Less_Tree implements Less_Tree_HasValueProperty {
 
 	public $key;
 	public $value;
diff --git a/lib/Less/Tree/Attribute.php b/lib/Less/Tree/Attribute.php
index f0f5d530..41c89acb 100644
--- a/lib/Less/Tree/Attribute.php
+++ b/lib/Less/Tree/Attribute.php
@@ -2,7 +2,7 @@
 /**
  * @private
  */
-class Less_Tree_Attribute extends Less_Tree {
+class Less_Tree_Attribute extends Less_Tree implements Less_Tree_HasValueProperty {
 
 	public $key;
 	public $op;
diff --git a/lib/Less/Tree/Call.php b/lib/Less/Tree/Call.php
index 3ce74edd..af0e95f0 100644
--- a/lib/Less/Tree/Call.php
+++ b/lib/Less/Tree/Call.php
@@ -3,7 +3,7 @@
  * @private
  * @see less.tree.Call in less.js 3.0.0 https://github.com/less/less.js/blob/v3.0.0/dist/less.js#L6336
  */
-class Less_Tree_Call extends Less_Tree {
+class Less_Tree_Call extends Less_Tree implements Less_Tree_HasValueProperty {
 	public $value;
 
 	public $name;
diff --git a/lib/Less/Tree/Comment.php b/lib/Less/Tree/Comment.php
index 8f4d8ee6..98a54366 100644
--- a/lib/Less/Tree/Comment.php
+++ b/lib/Less/Tree/Comment.php
@@ -2,7 +2,7 @@
 /**
  * @private
  */
-class Less_Tree_Comment extends Less_Tree {
+class Less_Tree_Comment extends Less_Tree implements Less_Tree_HasValueProperty {
 
 	public $value;
 	public $silent;
diff --git a/lib/Less/Tree/Dimension.php b/lib/Less/Tree/Dimension.php
index 7f3d063c..4c1e200d 100644
--- a/lib/Less/Tree/Dimension.php
+++ b/lib/Less/Tree/Dimension.php
@@ -2,7 +2,7 @@
 /**
  * @private
  */
-class Less_Tree_Dimension extends Less_Tree {
+class Less_Tree_Dimension extends Less_Tree implements Less_Tree_HasValueProperty {
 
 	public $value;
 	public $unit;
diff --git a/lib/Less/Tree/Directive.php b/lib/Less/Tree/Directive.php
index f5c88cf3..289f3942 100644
--- a/lib/Less/Tree/Directive.php
+++ b/lib/Less/Tree/Directive.php
@@ -2,7 +2,7 @@
 /**
  * @private
  */
-class Less_Tree_Directive extends Less_Tree {
+class Less_Tree_Directive extends Less_Tree implements Less_Tree_HasValueProperty {
 
 	public $name;
 	public $value;
diff --git a/lib/Less/Tree/Element.php b/lib/Less/Tree/Element.php
index 2c69bcc6..784cc6c8 100644
--- a/lib/Less/Tree/Element.php
+++ b/lib/Less/Tree/Element.php
@@ -2,7 +2,7 @@
 /**
  * @private
  */
-class Less_Tree_Element extends Less_Tree {
+class Less_Tree_Element extends Less_Tree implements Less_Tree_HasValueProperty {
 
 	/** @var string */
 	public $combinator;
diff --git a/lib/Less/Tree/Expression.php b/lib/Less/Tree/Expression.php
index 6e928d77..cdabf390 100644
--- a/lib/Less/Tree/Expression.php
+++ b/lib/Less/Tree/Expression.php
@@ -2,7 +2,7 @@
 /**
  * @private
  */
-class Less_Tree_Expression extends Less_Tree {
+class Less_Tree_Expression extends Less_Tree implements Less_Tree_HasValueProperty {
 	/** @var array */
 	public $value = [];
 	public $parens = false;
diff --git a/lib/Less/Tree/HasValueProperty.php b/lib/Less/Tree/HasValueProperty.php
new file mode 100644
index 00000000..5dbf9c51
--- /dev/null
+++ b/lib/Less/Tree/HasValueProperty.php
@@ -0,0 +1,8 @@
+<?php
+/**
+ * @private
+ * @property mixed $value
+ */
+interface Less_Tree_HasValueProperty {
+
+}
diff --git a/lib/Less/Tree/Keyword.php b/lib/Less/Tree/Keyword.php
index eea6bc70..0935667d 100644
--- a/lib/Less/Tree/Keyword.php
+++ b/lib/Less/Tree/Keyword.php
@@ -2,7 +2,7 @@
 /**
  * @private
  */
-class Less_Tree_Keyword extends Less_Tree {
+class Less_Tree_Keyword extends Less_Tree implements Less_Tree_HasValueProperty {
 
 	public $value;
 
diff --git a/lib/Less/Tree/NameValue.php b/lib/Less/Tree/NameValue.php
index 7a4bb9a1..7c9fc59a 100644
--- a/lib/Less/Tree/NameValue.php
+++ b/lib/Less/Tree/NameValue.php
@@ -10,7 +10,7 @@
  *
  * @private
  */
-class Less_Tree_NameValue extends Less_Tree {
+class Less_Tree_NameValue extends Less_Tree implements Less_Tree_HasValueProperty {
 
 	public $name;
 	public $value;
diff --git a/lib/Less/Tree/Negative.php b/lib/Less/Tree/Negative.php
index dc482e50..21289919 100644
--- a/lib/Less/Tree/Negative.php
+++ b/lib/Less/Tree/Negative.php
@@ -2,7 +2,7 @@
 /**
  * @private
  */
-class Less_Tree_Negative extends Less_Tree {
+class Less_Tree_Negative extends Less_Tree implements Less_Tree_HasValueProperty {
 
 	public $value;
 
diff --git a/lib/Less/Tree/Paren.php b/lib/Less/Tree/Paren.php
index ee1fd86a..8c9e139a 100644
--- a/lib/Less/Tree/Paren.php
+++ b/lib/Less/Tree/Paren.php
@@ -2,7 +2,7 @@
 /**
  * @private
  */
-class Less_Tree_Paren extends Less_Tree {
+class Less_Tree_Paren extends Less_Tree implements Less_Tree_HasValueProperty {
 
 	/** @var Less_Tree */
 	public $value;
diff --git a/lib/Less/Tree/Quoted.php b/lib/Less/Tree/Quoted.php
index ba3dcda5..eb6e107c 100644
--- a/lib/Less/Tree/Quoted.php
+++ b/lib/Less/Tree/Quoted.php
@@ -2,7 +2,7 @@
 /**
  * @private
  */
-class Less_Tree_Quoted extends Less_Tree {
+class Less_Tree_Quoted extends Less_Tree implements Less_Tree_HasValueProperty {
 	public $escaped;
 	public $value;
 	public $quote;
diff --git a/lib/Less/Tree/Rule.php b/lib/Less/Tree/Rule.php
index 38902d4e..c7daaea0 100644
--- a/lib/Less/Tree/Rule.php
+++ b/lib/Less/Tree/Rule.php
@@ -2,7 +2,7 @@
 /**
  * @private
  */
-class Less_Tree_Rule extends Less_Tree {
+class Less_Tree_Rule extends Less_Tree implements Less_Tree_HasValueProperty {
 
 	public $name;
 	/** @var Less_Tree */
diff --git a/lib/Less/Tree/Selector.php b/lib/Less/Tree/Selector.php
index ac3a525e..8b2517b0 100644
--- a/lib/Less/Tree/Selector.php
+++ b/lib/Less/Tree/Selector.php
@@ -99,7 +99,7 @@ public function CacheElements() {
 				continue;
 			}
 
-			if ( !property_exists( $v->value, 'value' ) || !is_string( $v->value->value ) ) {
+			if ( !( $v->value instanceof Less_Tree_HasValueProperty ) || !is_string( $v->value->value ) ) {
 				$this->cacheable = false;
 				return;
 			}
diff --git a/lib/Less/Tree/UnicodeDescriptor.php b/lib/Less/Tree/UnicodeDescriptor.php
index 03a4fb0b..cc1b39e1 100644
--- a/lib/Less/Tree/UnicodeDescriptor.php
+++ b/lib/Less/Tree/UnicodeDescriptor.php
@@ -2,7 +2,7 @@
 /**
  * @private
  */
-class Less_Tree_UnicodeDescriptor extends Less_Tree {
+class Less_Tree_UnicodeDescriptor extends Less_Tree implements Less_Tree_HasValueProperty {
 
 	public $value;
 
diff --git a/lib/Less/Tree/Url.php b/lib/Less/Tree/Url.php
index 9732f26f..140153b1 100644
--- a/lib/Less/Tree/Url.php
+++ b/lib/Less/Tree/Url.php
@@ -2,7 +2,7 @@
 /**
  * @private
  */
-class Less_Tree_Url extends Less_Tree {
+class Less_Tree_Url extends Less_Tree implements Less_Tree_HasValueProperty {
 
 	public $attrs;
 	public $value;
diff --git a/lib/Less/Tree/Value.php b/lib/Less/Tree/Value.php
index 437cf3d7..88c89849 100644
--- a/lib/Less/Tree/Value.php
+++ b/lib/Less/Tree/Value.php
@@ -2,7 +2,7 @@
 /**
  * @private
  */
-class Less_Tree_Value extends Less_Tree {
+class Less_Tree_Value extends Less_Tree implements Less_Tree_HasValueProperty {
 
 	public $value;