diff --git a/procs.tex b/procs.tex
index 5bbb178..34f32da 100644
--- a/procs.tex
+++ b/procs.tex
@@ -467,3 +467,61 @@ \section{Control features}
 ((compose - *) 3 4)              \ev  -12%
 \end{scheme}
 \end{entry}
+
+\section{Input and output}
+
+Pico Scheme allows output to a character device. Input is not
+specified in this report but an implementation may be extended to
+support input. Since expressions are side-effect free, standard Pico
+Scheme programs should not perform input or output from expressions
+and implementations can require that input or output from them is an
+error.  Output is only required to be allowed at the outermost level
+of a program.
+
+\begin{rationale}
+  Implementations may choose to allow input and output (IO) from
+  expressions (like \rsevenrs\ allows) or choose to forbid it.  An
+  implementation that forbids IO side-effects in expressions but
+  wishes to allow IO in places besides the outermost level would
+  likely need to extend Scheme in a way that is not compatible with
+  \rsevenrs.
+
+  For example, an implementation could add a command type that allowed
+  IO, and syntax to create it and then allow {\cf (define displayline
+    (delta (x) (display x) (newline)))} to define a new command {\cf
+    displayline}.
+
+  Implementations could add a non-expression {\cf do} that allows
+  IO inside it, similar to \rsevenrs's {\cf do} to support more
+  flexible IO while remaining a subset of \rsevenrs.
+
+  Besides {\cf display} and {\cf newline}, implementations could add
+  {\cf write-u8}, {\cf (define} \hyper{indentifier} {\cf (read))}, and
+  {\cf (define} \hyper{identifier} {\cf (read-u8))} to the outermost
+  level to support more IO. Other IO from \rsevenrs{} can be added if
+  string or char or port types are added.
+\end{rationale}
+
+\subsection{Output}
+\label{outputsection}
+
+\begin{entry}{%
+\proto{display}{ obj}{input or output}
+}
+
+Writes a representation of \var{obj} to the textual output.
+For booleans, nulls, numbers, symbols and pairs, this should
+be an external representation of the object.
+Returns an unspecified value.
+
+\end{entry}
+
+\begin{entry}{%
+\proto{newline}{}{input or output}
+}
+
+Writes an end of line to textual output.  Exactly how this
+is done differs
+from one operating system to another. Returns an unspecified value.
+
+\end{entry}
diff --git a/prog.tex b/prog.tex
index bda6396..2724f87 100644
--- a/prog.tex
+++ b/prog.tex
@@ -4,7 +4,7 @@ \chapter{Program structure}
 \section{Programs}
 
 A Scheme program consists of a sequence of
-expressions and definitions.
+expressions, definitions and output.
 Expressions are described in chapter~\ref{expressionchapter}.
 Definitions are variable definitions which are explained in this chapter.
 They are valid in some, but not all, contexts where expressions
diff --git a/struct.tex b/struct.tex
index ddbb7b5..6c1f551 100644
--- a/struct.tex
+++ b/struct.tex
@@ -57,7 +57,8 @@ \subsection{Base and optional features}
 Pico Scheme is already reduced, but if a smaller subset is desired,
 either symbols or integers could be removed. Either {\cf cond} or {\cf
   if} could be removed. Both {\cf let} and {\cf apply} could be
-removed. If extended, it is recommend to use \rsevenrs\ as a
+removed. If only a REPL is provided, output could be removed.
+If extended, it is recommend to use \rsevenrs\ as a
 guide. For cases where both \rsevenrs\ and Pico Scheme are using
 defined behavior, it is intended that Pico Scheme should have
 identical results.
@@ -178,6 +179,7 @@ \subsection{Naming conventions}
 By convention, \ide{?} is the final character of the names
 of procedures that always return a boolean value.
 Such procedures are called \defining{predicates}.
-Predicates are generally understood to be side-effect free, except that they
+Predicates, like all procedures in Pico Scheme, are generally
+side-effect free, except that they
 may have an error when passed the wrong type of argument.
 
diff --git a/syn.tex b/syn.tex
index 8780822..584efc0 100644
--- a/syn.tex
+++ b/syn.tex
@@ -156,8 +156,9 @@ \subsection{Programs and definitions}
 
 \begin{grammar}%
 \meta{program} \:
-\> \atleastone{\meta{expression or definition}}
-\meta{expression or definition} \: \meta{expression}
-\> \| \meta{definition}
+\> \atleastone{\meta{expression or definition or io}}
+\meta{expression or definition or io} \: \meta{expression}
+\> \| \meta{definition} \| \meta{io}
 \meta{definition} \: (define \meta{identifier} \meta{expression})
+\meta{io} \: (display \meta{identifier}) \| (newline)
 \end{grammar}