reserve X for set;
reserve a,b,c,k,m,n for Nat;
reserve i,j for Integer;
reserve r,s for Real;
reserve p,p1,p2,p3 for Prime;

theorem Th56:
  20|^2 - 1 = 3*7*19
  proof
    20|^2 = 20*20 by WSIERP_1:1;
    hence thesis;
  end;
