[1] Lexicographical_compare_3way is almost, but not quite, redundant: the call lexicographical_compare_3way(f1,l1, f2,l2) could be written as lexicographical_compare(f1,l1, f2,l2) ? –1 : (lexicographical_compare(f2,l2, f1,l1) ? 1 : 0). The single call to lexicographical_compare_3way, however, is much faster than the two calls to lexicographical_compare.

[2] "Equivalent", not "equal", because two equivalent elements (that is, two elements with the property that neither one is less than the other) are not necessarily equal. Operator< is required to induce a strict weak ordering, not necessarily a total ordering. See the LessThan Comparable requirements for a discussion.