eval: implement not() expression

doc/eval.texi

@@ -76,6 +76,9 @@ integer. For example, "trunc(-1.5)" is "-1.0".
 @item sqrt(expr)
 Compute the square root of @var{expr}. This is equivalent to
 "(@var{expr})^.5".
+
+@item not(expr)
+Return 1.0 if @var{expr} is zero, 0.0 otherwise.
 @end table
 
 Note that:
@@ -93,11 +96,6 @@ is equivalent to
 A*B + not(A)*C
 @end example
 
-When A evaluates to either 1 or 0, that is the same as
-@example
-A*B + eq(A,0)*C
-@end example
-
 In your C code, you can extend the list of unary and binary functions,
 and define recognized constants, so that they are available for your
 expressions.