reserve c,c1,c2,x,y,z,z1,z2 for set;
reserve C1,C2,C3 for non empty set;

theorem
  for X,Y being non empty set for x being Element of X, y being Element
  of Y for f being RMembership_Func of X,Y holds (converse f).(y,x) = f.(x,y)
    by Def1,ZFMISC_1:87;
