A minimal theory to enable

This `theory` (see theories) enables only a few
built-in functions and executable-counterparts. It can be useful when
you want to formulate lemmas that rather immediately imply the theorem to be
proved, by way of a

:use (lemma-1 lemma-2 lemma-3) :in-theory (union-theories '(f1 f2) (theory 'minimal-theory))

In this example, we expect the current goal to follow from lemmas
`implies` and the `executable-counterpart` of
`car`). The `in-theory` hint above is intended to speed up
the proof by turning off all inessential rules.