Add semigroup-ish operation for printing sets (non UNI relations)

[sjcjoosten]

Consider the following problem. Given (only) a relation subconcept, how can we print it using this syntax:

CLASSIFY conceptname IS subc_1 /\ subc_2 /\ subc_3 /\ ... /\ subc_n

(We may assume subConcept is transitive reflexive and antisymmetric, so subc_i = conceptname)

The intended solution uses a relation combine :: (String,String) * String that, for this case, is specified by the function \v w -> v <> " /\\ " <> w.
Given concat :: (String,String) -> String it should be possible to define combine.

In order to show that the show relation is a function (up to equivalent outputs), we would need that combine is an abelian semigroup (up to equivalent outputs), hence the title of this ticket.

[sjcjoosten]

Since there is a SEPBY right now, I'm removing the "ready" label: an arbitrary operation seems more involved