Let f be an irreducible polynomial in K[x]. If f contains only one variable then x need not be specified, otherwise both K and x must be specified. If the argument K is not specified then K is the smallest extension of the rationals such that the coefficients of f are in K. If K is specified then the field K contains the RootOfs in this set as well. Let L be the field extension of K given by one single root of f. So L is not the splitting field; L = K[x]/(f) = K(RootOf(f,x). The call evala(Subfields(f, deg, K, x)) computes the set of all subfields of L over K of degree deg. Each subfield is given by a single RootOf of degree deg.