Describe a p:message step

[ndw]

It seems like a consensus is forming to add a p:message step, so I took a stab at describing it.

The only open question in my mind is, if the test expression is false(), is the implementation forbidden from evaluating the select expression or is that implementation-dependent. (Or implementation-defined, I guess.)

[ndw]

Fix #672

[xml-project]

It seems like a consensus is forming to add a p:message step, so I took a stab at describing it.

The only open question in my mind is, if the test expression is false(), is the implementation forbidden from evaluating the select expression or is that implementation-dependent. (Or implementation-defined, I guess.)

I am not sure we need to say something special here. There are other cases in evaluating a pipeline where a processor knows that an option is not used and therefore does not have to be evaluated, e.g. if match in p:add-attribute does not match anything, attribute-name and attribute-value don't have to be evauated.

The difference in user experience is (a) enhanced speed and (b) missing error messages resulting from the non evaluated expressions, right? I would think this could be implementation-dependent.