25. Computer apparatus according to claim 24 wherein the expression tree includes leaf nodes having associated graphical elements and internal nodes having associated codes for combining graphical elements, and wherein the portions include internal nodes and said means for estimating for the cost of execution of the alternative forms comprises: first means for determining a size of descendant graphical elements corresponding to descendants of a current node, and means for determining a cost from the size of the descendant graphical elements.
27. Computer apparatus according to claim 25, wherein at least one of the associated codes of the internal nodes is commutative and the alternative forms of the expression tree include commutative permutations of the portions.