removes mutable (idk why it was ever there)
removes mutable (idk why it was ever there)

jkhoogland/cppitertools,ryanhaining/cppitertools,ryanhaining/cppitertools,jkhoogland/cppitertools,tempbottle/cppitertools,tempbottle/cppitertools,jkhoogland/cppitertools,ryanhaining/cppitertools,tempbottle/cppitertools