reserve i,j,k,x for object;

theorem
  for A,B being set, F being ManySortedSet of [:B,A:], C being Subset of
A, D being Subset of B, x,y being set st x in C & y in D holds F.(y,x) = (F|([:
  D,C:] qua set)).(y,x) by FUNCT_1:49,ZFMISC_1:87;
