generalized optional dimensions to sequence of dimensions to allow double-indexing on LHS of assignment
generalized optional dimensions to sequence of dimensions to allow double-indexing on LHS of assignment

stan-dev/math,stan-dev/stan,stan-dev/stan,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/stan,stan-dev/math,stan-dev/math,stan-dev/stan,stan-dev/math,stan-dev/math,stan-dev/stan,stan-dev/math