9.5.12 `set_field'
------------------

set_field(ROOTLIST)
     :: Set an algebraic number field as the currernt ground field.

RETURN
     0

ROOTLIST
     A list of `root'

   * `set_field()' sets an algebraic number field generated by `root' in
     ROOTLIST over Q.

   * You don't care about the order of `root' in ROOTLIST, because
     `root' are automatically ordered internally.

     [0] A=newalg(x^2+1);
     (#0)
     [1] B=newalg(x^3+A);
     (#1)
     [2] C=newalg(x^4+B);
     (#1)
     [3] set_field([C,B,A]);
     0

Reference
     *Note `algtodalg dalgtoalg dptodalg dalgtodp': algtodalg dalgtoalg
     dptodalg dalgtodp.

