Function: polsubcyclo
Section: polynomials
C-Name: polsubcyclo
Prototype: LLDn
Help: polsubcyclo(n,d,{v=x}): finds an equation (in variable v) for the d-th
 degree subfields of Q(zeta_n). Output is a polynomial or a vector of
 polynomials is there are several such fields, or none.

