Description: Define the class of left ideals of a given ring. An ideal is a
submodule of the ring viewed as a module over itself. For the usual
textbook definition of a (left) ideal of a ring to be a subgroup of the
additive group of the ring which is closed under left-multiplication by
elements of the full ring, see dflidl2 . (Contributed by Stefan
O'Rear, 31-Mar-2015)