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 dflidl2rng and dflidl2 . (Contributed by Stefan
O'Rear, 31-Mar-2015)