let s1 = n(NT_LET, n(NT_VAR, id("n")),
                     n(NT_VAR, id("fac")), 
                     n(NT_VAR, id("i")));
  let s2 = n(NT_ASSIGN, n(NT_VAR, id("n")),
                        n(NT_NUMBER, 7));
  let s3 = n(NT_ASSIGN, n(NT_VAR, id("fac")),
                        n(NT_NUMBER, 1));
  let s4 = n(NT_ASSIGN, n(NT_VAR, id("i")),
                        n(NT_NUMBER, 1));
  let s5 = n(NT_ASSIGN, n(NT_VAR, id("fac")),
                        n(NT_MUL, n(NT_VAR, id("fac")),
                                  n(NT_VAR, id("i"))));
  let s6 = n(NT_ASSIGN, n(NT_VAR, id("i")),
                        n(NT_ADD, n(NT_VAR, id("i")),
                                  n(NT_NUMBER, 1)));
  let s7 = n(NT_WHILE, n(NT_LESSEQ, n(NT_VAR, id("i")),
                                    n(NT_VAR, id("n"))),
                       n(NT_SEQ, s5, s6));
  let s = n(NT_SEQ, s1, s2, s3, s4, s7);

/*
  equivalent to
    { let n, fac, i;
      n := 7;
      fac := 1;
      i := 1;
      while i <= n do
      { fac := fac * i;
        i := i - 1 } }
*/