reserve V for non empty set,
  A,B,A9,B9 for Element of V;
reserve f,f9 for Element of Funcs(V);
reserve m,m1,m2,m3,m9 for Element of Maps V;

theorem Th13:
  dom m2 = cod m1 & dom m3 = cod m2 implies m3*(m2*m1) = (m3*m2)* m1
proof
  assume that
A1: dom m2 = cod m1 and
A2: dom m3 = cod m2;
A3: cod(m2*m1) = cod m2 by A1,Th12;
  (m2*m1)`2 = m2`2*m1`2 by A1,Th12;
  then
A4: (m3*(m2*m1))`2 = m3`2*(m2`2*m1`2) by A2,A3,Th12;
A5: dom(m3*m2) = dom m2 by A2,Th12;
  then
A6: dom((m3*m2)*m1) = dom m1 by A1,Th12;
  dom(m2*m1) = dom m1 by A1,Th12;
  then
A7: dom(m3*(m2*m1)) = dom m1 by A2,A3,Th12;
  cod(m3*m2) = cod m3 by A2,Th12;
  then
A8: cod((m3*m2)*m1) = cod m3 by A1,A5,Th12;
  (m3*m2)`2 = m3`2*m2`2 by A2,Th12;
  then
A9: ((m3*m2)*m1)`2 = (m3`2*m2`2)*m1`2 by A1,A5,Th12;
  cod(m3*(m2*m1)) = cod m3 by A2,A3,Th12;
  hence thesis by A4,A7,A9,A6,A8,Lm2,RELAT_1:36;
end;
