  Lickorish has constructed large families of contractible 4--manifolds that
have knotted embeddings in the 4--sphere and has also shown that every finitely
presented perfect group with balanced presentation occurs as the fundamental
group of the complement of a knotted contractible manifold. Here we make a few
observations regarding Lickorish's construction, showing how to extend it to
construct contractible 4--manifolds which have an infinite number of knotted
embeddings and also to construct knotted embeddings of the Mazur manifold for
which the complement has trivial fundamental group.
