Simplify callable from a to l #236

Job	Run time
macosx (-O0)	40m 30s
linux (g++, -O3 -DNDEBUG)	14m 32s
macosx (-O0 -stdlib=libc++, -stdlib=libc++)	38m 24s
linux (clang++, -O3 -DNDEBUG)	22m 37s
macosx (-O3 -flto -DNDEBUG)	24m 26s
linux (g++, -O3 -DNDEBUG -mavx2)	12m 41s
linux (clang++, -O3 -DNDEBUG -mavx2)	20m 18s
linux (g++, -O3 -DNDEBUG -march=skylake-avx512)	12m 30s
macosx (-O3 -flto -DNDEBUG -stdlib=libc++, -stdlib=libc++)	24m 11s
linux (clang++, -O3 -DNDEBUG -march=skylake-avx512)	18m 2s
	3h 48m 11s

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