Selection vs. typing to specify source/target elements involved in mappings seems unintuitive

[jpav]

Currently, mappings can be created very quickly via two clicks, one on element in the source tree and one in the target tree. Editing existing mappings can also be accomplished via simple tree selection. However, mappings that involve slightly more complicated transformations, such as the combine and separate actions, involve multiple source/target elements. In these cases, the user cannot use selection to add these additional elements to the transformation, but rather must type their names in text fields within the mapping detail area. The need to switch from selecting elements to typing their names seems unintuitive and likely to lead the user to inadvertently creating unwanted new mappings when trying to continue to use the selection paradigm when editing existing mappings. One suggestion would be to consider sacrificing a bit of the speed with simple mappings by introducing an explicit "map" button after the initial source/target selections are made to new mappings. This would allow for a similar flow when editing the aforementioned more advanced mappings such that the user could continue to use selection within the trees and simply click on buttons (presumably next to the associated fields) in the mapping detail area to update the text fields with those selections. The nice part is this would not preclude the use of the existing typing approach to add elements.