scheme :: WAYBEL_2:sch 1
ExNet{ F1() -> non empty RelStr , F2() -> prenet of F1(), F3( set ) -> Element of F1() } :
ex M being prenet of F1() st
( RelStr(# the carrier of M, the InternalRel of M #) = RelStr(# the carrier of F2(), the InternalRel of F2() #) & ( for i being Element of M holds the mapping of M . i = F3(( the mapping of F2() . i)) ) )