reserve S for non empty non void ManySortedSign;
reserve X for non-empty ManySortedSet of S;
reserve x,y,z for set, i,j for Nat;

theorem Th27:
  for A1,A2 being MSAlgebra over S
  st the MSAlgebra of A1 = the MSAlgebra of A2
  for B1 being MSSubset of A1, B2 being MSSubset of A2
  st B1 = B2 & B1 is opers_closed holds B2 is opers_closed
  by Th24;
