
theorem Th5:
  1.(Z/2) + 1.(Z/2) = 0.(Z/2)
   proof
A1:  Z/2 =
     doubleLoopStr(#Segm(2),addint(2),multint(2),In(1,Segm(2)), In(0,Segm(2))#)
     by INT_3:def 12;
     1.(Z/2) = 1 by INT_3:14;
     hence 1.(Z/2) + 1.(Z/2) = (1+1) mod 2 by A1,GR_CY_1:def 4
     .= 0 by INT_1:50 .= 0.(Z/2) by Th4;
   end;
