I want to propose an alternative way to solve issue 157, without
introducing significant complexity to the language.
The XSLT specification has been dealing with the exact same problem of
evaluating an expression and placing the results in a target context.
XSLT provides two separate mechanisms: copy-of and value-of. I would
propose using these two XSLT elements as a normative part of the BPEL
specification, instead of inventing yet another copy mechanism.