doc/concepts.tex: Explicitly define `instances' clearly.

diff --git a/doc/concepts.tex b/doc/concepts.tex
index d9239d0..58f781a 100644 (file)
--- a/doc/concepts.tex
+++ b/doc/concepts.tex
@@ -63,22 +63,33 @@ having to implement additional syntax.
 For the most part, Sod takes a fairly traditional view of what it means to be
 an object system.
 
-An \emph{object} maintains \emph{state} and exhibits \emph{behaviour}.  An
-object's state is maintained in named \emph{slots}, each of which can store a
-C value of an appropriate (scalar or aggregate) type.  An object's behaviour
-is stimulated by sending it \emph{messages}.  A message has a name, and may
-carry a number of arguments, which are C values; sending a message may result
-in the state of receiving object (or other objects) being changed, and a C
-value being returned to the sender.
-
-Every object is a (direct) instance of some \emph{class}.  The class
-determines which slots its instances have, which messages its instances can
-be sent, and which methods are invoked when those messages are received.  The
-Sod translator's main job is to read class definitions and convert them into
-appropriate C declarations, tables, and functions.  An object cannot
+An \emph{object} maintains \emph{state} and exhibits \emph{behaviour}.
+(Here, we're using the term `object' in the usual sense of `object-oriented
+programming', rather than that of the ISO~C standard.  Once we have defined
+an `instance' below, we shall generally prefer that term, so as to prevent
+further confusion between these two uses of the word.)
+
+An object's state is maintained in named \emph{slots}, each of which can
+store a C value of an appropriate (scalar or aggregate) type.  An object's
+behaviour is stimulated by sending it \emph{messages}.  A message has a name,
+and may carry a number of arguments, which are C values; sending a message
+may result in the state of receiving object (or other objects) being changed,
+and a C value being returned to the sender.
+
+Every object is a \emph{direct instance} of exactly one \emph{class}.  The
+class determines which slots its instances have, which messages its instances
+can be sent, and which methods are invoked when those messages are received.
+The Sod translator's main job is to read class definitions and convert them
+into appropriate C declarations, tables, and functions.  An object cannot
 (usually) change its direct class, and the direct class of an object is not
 affected by, for example, the static type of a pointer to it.
 
+If an object~$x$ is a direct instance of some class~$C$, then we say that $C$
+is \emph{the class of}~$x$.  Note that the class of an object is a property
+of the object's value at runtime, and not of C's compile-time type system.
+We shall be careful in distinguishing C's compile-time notion of \emph{type}
+from Sod's run-time notion of \emph{class}.
+
 
 \subsection{Superclasses and inheritance}
 \label{sec:concepts.classes.inherit}
@@ -120,7 +131,7 @@ class then the only superclass of $C$ is $C$ itself, and $C$ has no proper
 superclasses.
 
 If an object is a direct instance of class~$C$ then the object is also an
-(indirect) instance of every superclass of $C$.
+(indirect) \emph{instance} of every superclass of $C$.
 
 If $C$ has a proper superclass $B$, then $B$ must not have $C$ as a direct
 superclass.  In different terms, if we construct a directed graph, whose