reserve x,y for Real;
reserve z,z1,z2 for Complex;
reserve n for Element of NAT;

theorem
  cosh_C/.0c = 1
proof
  cosh_C/.0c = (exp 0c + exp(-0c))/2 by Def4
    .=1 by SIN_COS:49,51;
  hence thesis;
end;
