+-- If we have
+-- lcl_id = e; exp_id = lcl_id
+-- and lcl_id has useful IdInfo, we don't want to discard it by going
+-- gbl_id = e; lcl_id = gbl_id
+-- Instead, transfer IdInfo from lcl_id to exp_id
+-- Overwriting, rather than merging, seems to work ok.