Be able to define user-defined functions within DA...

Currently, user-defined functions are defined at the model level. It would be nice to be able to define functions that are local to a DAX expression (e.g. the DAX expression of a measure, calculated column, or even a variable) so that the model-level scope does not get cluttered with functions that are needed in only a few places. This might require treating functions as first-class citizens, similar to what was done in Excel with LAMBDA, so that functions can be assigned to variables.