LWG3472 counted_iterator is missing preconditions

[iterators.counted] Add "is true" for expressions in preconditions.

Author: burblebee

source/iterators.tex

@@ -5338,6 +5338,10 @@
 \end{itemdecl}
 
 \begin{itemdescr}
+\pnum
+\expects
+\tcode{length > 0} is \tcode{true}.
+
 \pnum
 \effects
 Equivalent to: \tcode{return *current;}
@@ -5641,6 +5645,10 @@
 \end{itemdecl}
 
 \begin{itemdescr}
+\pnum
+\expects
+\tcode{i.length > 0} is \tcode{true}.
+
 \pnum
 \effects
 Equivalent to: \tcode{return ranges::iter_move(i.current);}
@@ -5655,6 +5663,10 @@
 \end{itemdecl}
 
 \begin{itemdescr}
+\pnum
+\expects
+Both \tcode{x.length > 0} and \tcode{y.length > 0} are \tcode{true}.
+
 \pnum
 \effects
 Equivalent to \tcode{ranges::iter_swap(x.current, y.current)}.