Description: Define the canonical injection from symbols to words. Although not
required, A should usually be a set. Otherwise, the singleton word
<" A "> would be the singleton word consisting of the empty set, see
s1prc , and not, as maybe expected, the empty word, see also s1nz .
(Contributed by Stefan O'Rear, 15-Aug-2015)(Revised by Mario Carneiro, 26-Feb-2016)