Mutable lexeme

A mutable lexeme is a complex lexeme with one or more variable constituents.

Example
"The mother of all battles", Saddam Hussein's variant of a common Arabic expression, entered English and immediately became mutable, as exemplified by the expression "the mother of all meteors", which was used by the New York Times when reporting a spectacular meteor seen over the Eastern United States.

Comments
Some mutable lexemes have more than one variable, for example, "you don't have to be a X to Y that Z", where X = "brain surgeon" or "rocket scientist", Y = "understand" or "appreciate" or "see". Z varies over an infinite range of possible clauses, exemplifying a broad sort of variable that is not at all uncommon.

What linguists have customarily called constructions can all be viewed as mutable lexemes. Many of them have one or more fixed constituents. For example, the "way" construction has the one fixed constituent "way", plus several variable constituents. From a TV news broadcast during the energy crisis a few years ago, we have, "...whether Californians can conserve their way out of this crisis...". Note that "conserve" is in no way a verb of motion. The (metaphorical) motion element of the meaning is imparted by the construction (i.e., mutable lexeme) itself.

From the fact that all constructions are mutable lexemes, it follows that the whole of a lexicogrammatical system is a collection of lexemes, some fixed, some mutable.