  An algorithm is proposed that solves two decision problems for pseudo-Anosov
elements in the mapping class group of a surface with at least one marked fixed
point. The first problem is the root problem: decide if the element is a power
and in this case compute the roots. The second problem is the symmetry problem:
decide if the element commutes with a finite order element and in this case
compute this element. The structure theorem on which this algorithm is based
provides also a new solution to the conjugacy problem.
