Checks whether a given evar is a blank (entered by the user with the `_` syntax) in the evar_map.

Refines the current goal with just a new named evar, the name of which is based on the input string. The use of this is to replace unnamed evars with named ones, so that the user can refer to them later.

A tactic that resturns a list of all evars in a term (= Evd.econstr) that were introduced by the user as a blank and have not been resolved yet.