Not used by SableCC-generated code (yet?), but may be used by other code
that modifies lexer/parser exceptions.

This matches the comments at the top of all of the sablecc-runtime
source files. I'm also leaving in the LGPLv2.1 as an alternative license
though - according to the FSF, the EPLv1.0 is incompatible with GPL
licenses, so the dual license is necessary to allow distributing our
SableCC fork. (All code in sablecc-runtime was written by us, so we're
free to relicense it like this.)

Unfortunately, the check causes errors in our EventBLexer and this isn't
easy to fix.

Our parsers actually do use this method in a few cases that aren't easy
to replace.

Now that this method is only called for newly created AST nodes, there's
no need anymore to handle node lists, tokens, and nodes that already
have positions.