\version "2.19.12"

ext = #(define-music-function (parser location voice syl) (symbol? ly:music?)
         (let*
          ((s? (eq? voice 'S))
           (a? (eq? voice 'A))
           (t? (eq? voice 'T))
           (b? (eq? voice 'B))
           (others (cond (s? '(A T B))
                     (a? '(S T B))
                     (t? '(S A B))
                     (b? '(S A T)))))
          #{ \lyricmode { \tag $voice { $syl __ }
             \tag $others $syl } #}))

music = << { c( d) e f }
           \addlyrics { \ext A Will it work? } >>

\keepWithTag A \music
\keepWithTag B \music