Here, in the Interpreter, I have extracted the code for
executing each Abstract Pattern Instruction into a
separate runXXX() function [runAt(), runMatch(), runTest()
and runCapture()].

I vaguely thought of this as preparation for integrating
the Translator and Interpreter later.

HOWEVER, in fact it's not terribly useful, because the
integration will be much more intertwined - specifically
because runMatch() calls interprete() recursively.