[firedrake] Expressions compiler

Dear Firedrakers, Please excuse my excitement, but the expressions compiler is extremely cool. For example, it allows an application of an SSPRK timestepping scheme in the following compact form: while (t <= end): solve(lhsA == action(rhsA,oldT), newT) T1.assign(newT) solve(lhsA == action(rhsA,T1), newT) T2.assign(0.75*oldT + 0.25*newT) solve(lhsA == action(rhsA,T2), newT) newT.assign((1.0/3.0)*oldT + (2.0/3.0)*newT) oldT.assign(newT) cheers --cjc