Some mention should be made of "extensible" languages, a trend that seems to have reached its height with two symposia circa 1970, [Chri 69, Schu 71]. The general idea was that the programmer would be given the means to construct a specialized language for any arbitrary application area by extending a base language. In practice, the name "extensible languages" was applied to everything from SIMULA, to PL/I (because of its bewildering macro facility), to compiler-compilers. The trend lost momentum after the symposia; see [Stan 75].
Although the concept of extensible languages appears quite similar to the adaptivity principle, very little of the research done was accompanied by formal grammar models. An exception is Wegbreit's ECFGs; W-grammars might also be excepted, since ALGOL 68 was painted as an extensible language in the earlier of the two symposia. Also, a pathological case was the extensible language AEPL, in which the language processor was driven by a dynamically modifiable set of Chomsky type 0 grammar rules with programs attached to them.
See also: Wikipedia article "Extensible programming".