1. A method of evaluating an expression tree, the method comprising: at a first domain, accessing an expression tree, wherein the expression tree comprises a plurality of nested expressions, the plurality of nested expressions spanning a plurality of domains, but wherein the expressions in the expression tree are expressed in a common format in spite of the different expressions of the expression tree being intended to be evaluated by different domains; at the first domain, evaluating one or more references to local variables included in the plurality of nested expressions; determining that one or more expressions in the plurality of expressions are to be evaluated by a different domain; and expanding the one or more expressions in the plurality of expressions to be evaluated by different domains to allow the one or more expressions to be evaluated by the different domain.