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\rput{0}(55,09){\Large\bf $a$}
\rput{0}(75,10){\Large\bf $b$}
\rput{0}(95,09){\Large\bf $c$}

\ss
{\it {\bf Fig 1}\pointir Representation of a partially commutative monomial by ``heaps of pieces'' \cite{Vi}. Here ``$a$'' commutes with ``$c$'' (they are on non-concurrent positions) and ``$b$'' commutes with no other piece. The monomial represented can be written by several words as $acbc^3ba^2c=cabc^3baca=cabc^3bca^2$.}
