what to do about left-shifted first terms?

[mengwong]

normally we expect to see

WHO walks
AND runs
AND    degustates
    OR imbibes

what happens if we have something of the form

WHO walks
AND runs
AND degustates
    OR imbibes

we are probably good for constitutive rules

WHO walks
AND runs
AND degustates
    MEANS eats
       OR drinks

but we need to figure out the semantics for a boolrule

or just prohibit it as a syntax error, so we need a notion of minimum depth

[mengwong]

i have added some tests for constitutive rules with left-shifted first terms, but this is not super interesting.