It is shown how ideas adapted from recent work on explanation-based generalization can be used to allow a logic grammar to "learn" useful derived grammar rules by generalizing them from example sentences. The method is presented in the form of a small Prolog meta-interpreter, and its soundness is formally proved. Examples are given showing the application of the generalizer, first to a toy grammar with 40 rules and then to a largish independantly developed system which involves non-trivial syntactic and semantic analysis.