nice man: selecting between lexeme and another that is its prefix

[mmoskal]

The nice man example now works. The following example fails:

    g = select(["a", "abq", "c"]) + optional("bQ")
    check_grammar(g, ["", "a‧b‧q‧≺EOS≻"]) # fails 'q' is forced
    check_grammar(g, ["", "a‧b‧Q"]) # doesn't match

cc @hudson-ai