  Vladimir Shpilrain and Jie-Tai Yu have asked for an effective algorithm to
decide if two elements of C[x,y] are related by an automorphism of C[x,y]. We
describe here an efficient algorithm that decides this question and finds the
automorphism if it exists. The algorithm is due to the second author, who
described it in terms of Newton polygons, with C replaceable by a field of any
characteristic. Here we describe it in terms of splice diagrams, which gains
some efficiency at cost of generality (it currently applies only to
characteristic zero). Part of the purpose of this paper is to give an
exposition of the use of splice diagrams in studying C[x,y].
