When a keyword is being handled, the plugin function must build
a tree of C<OP> structures, representing the code that was parsed.
The root of the tree must be stored in C<*op_ptr>. The function then
-returns a contant indicating the syntactic role of the construct that
+returns a constant indicating the syntactic role of the construct that
it has parsed: C<KEYWORD_PLUGIN_STMT> if it is a complete statement, or
C<KEYWORD_PLUGIN_EXPR> if it is an expression. Note that a statement
construct cannot be used inside an expression (except via C<do BLOCK>