.why_not mis-evaluates views whose rules have constants in the head, reporting derived rows as not derived #291

>>> ?candidate("s-43", T)
<<< 3 row(s): escalate, issue_refund, open_ticket                  # the row exists
>>> .why_not allowed("s-43", "issue_refund")
<<< "Blocker: Body predicate 0 (candidate(\"s-43\", \"issue_refund\")) failed: No matching tuples in candidate"
>>> .why_not allowed("s-43", "open_ticket")                         # this row IS derived (see ?allowed above)
<<< "allowed(\"s-43\", \"open_ticket\") was NOT derived: ... No matching tuples in candidate"
>>> .why_not candidate("s-43", "issue_refund")
<<< clause 1 and 2: "Head unification failed: Target arity 2 does not match head arity 2"