M. Petrich et Cm. Reis, THE SYNTACTIC MONOID OF THE SEMIGROUP GENERATED BY A COMMA-FREE CODE, Proceedings of the Royal Society of Edinburgh. Section A. Mathematics, 125, 1995, pp. 165-179
A sequence of lemmas leads to a two-fold characterisation of the synta
ctic monoid in the title. Some alternatives as well as special cases,
in particular when the code consists of a singleton, are considered.