hedge.tools.SubsettableCrossProduct:
A cross product that can operate on an arbitrary subsets of its two
operands and return an arbitrary subset of its result.
hedge.optemplate.PrioritizedSubexpression:
When the optemplate-to-code transformation is performed,
prioritized subexpressions work like common subexpression in that
they are assigned their own separate identifier/register location.
hedge.optemplate.LocalOpReducerMixin:
Reduces calls to mapper methods for all local differentiation
operators to a single mapper method, and likewise for mass
operators.