Lvalue Iterator Concept

The Lvalue Iterator concept adds the requirement that the return type of operator* type be a reference to the value type of the iterator.

Lvalue Iterator Requirements
Expression	Return Type	Note/Assertion
`*a`	`T&`	`T` is cv `iterator_traits<X>::value_type` where cv is an optional cv-qualification. pre: `a` is dereferenceable. If `a == b` then `a` is equivalent to `b`.

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