theorem Th33:
  Solutions_of(A,B) is non empty implies len A = len B
proof
  assume Solutions_of(A,B) is non empty;
  then consider x being object such that
A1: x in Solutions_of(A,B);
  ex X st X=x & len X= width A & width X = width B & A * X = B by A1;
  hence thesis by MATRIX_3:def 4;
end;
