; let (clas, inst_tys) = tcSplitDFunHead inst_head
(class_tyvars, sc_theta, _, op_items) = classBigSig clas
sc_theta' = substTheta (zipOpenTvSubst class_tyvars inst_tys) sc_theta
; let (clas, inst_tys) = tcSplitDFunHead inst_head
(class_tyvars, sc_theta, _, op_items) = classBigSig clas
sc_theta' = substTheta (zipOpenTvSubst class_tyvars inst_tys) sc_theta