 reserve x for object;
 reserve G for non empty 1-sorted;
 reserve A for Subset of G;
 reserve y,y1,y2,Y,Z for set;
 reserve k for Nat;
 reserve G for Group;
 reserve a,g,h for Element of G;
 reserve A for Subset of G;
reserve G for non empty multMagma,
  A,B,C for Subset of G;
reserve a,b,g,g1,g2,h,h1,h2 for Element of G;

theorem Th28:
  x in A * g iff ex h st x = h * g & h in A
proof
  thus x in A * g implies ex h st x = h * g & h in A
  proof
    assume x in A * g;
    then consider g1,g2 such that
A1: x = g1 * g2 & g1 in A and
A2: g2 in {g};
    g2 = g by A2,TARSKI:def 1;
    hence thesis by A1;
  end;
  given h such that
A3: x = h * g & h in A;
  g in {g} by TARSKI:def 1;
  hence thesis by A3;
end;
