more robust (amending 8f53fa93d5f0): R could be anything and Thm.instantiate' involves some type-checks, e.g. relevant for lemma fset_simps in theory Quotient_Examples.Quotient_FSet;
src/Pure
src/FOL
src/HOL
src/ZF
src/CCL
src/CTT
src/Cube
src/FOLP
src/LCF
src/Sequents
src/Doc
src/Tools