alloy* is ok, but alloy reject this. since "Analysis cannot be performed since it requires higher-order quantification that could not be skolemized."

HOQ.als

sig A {
}
sig B {
  S : set A
}

sig C {
	R : set B
}

pred Monic(m : S){
	all f,g: R | f.m = g.m <=> f = g
}

pred Epic(e : R){
	all f,g: S | e.f = e.g <=> f = g
}

pred show{
	some m:S | Monic[m]
	some e:R | Epic[e]
}

run show