Creates a predicate abstraction sumti out of a full bridi clause, binding all the necessary lambda variables to the ko'a-ko'u pronoun series. The number of bound variables must be indicated by appending {xi} followed by that number to the word {fo'ai}, unless only one variable (namely {fo'a}) is bound, in which case the {xi} marking is optional.
Description clause: create a sumti from the enclosed bridi, describing the referent of the created sumti as filling the bridi place filled with {ke'a}.