theorem Th143:
  len f = len sqr f & dom f = dom sqr f
proof
  rng f c= REAL & dom sqrreal = REAL by FUNCT_2:def 1;
  hence len f = len (sqrreal*f) by FINSEQ_2:29
    .= len sqr f;
  hence thesis by FINSEQ_3:29;
end;
