Trace monoids (also called heaps of pieces monoids, Cartier-Foata monoids or free partially commutative monoids) are defined by the presentation M = <S | ab = ba if (a, b) [member of] I>, where S is a finite set of generators and I is a symmetric and antireflexive
relation on S x S called the commutation relation.
possibly annotated by some subscript) to refer to strict partial orders (antisymmetric, antireflexive
, and transitive binary relations) and [predecessor of equal to] and ?
The crowd is represented essentially, in an antireflexive
movement, and what lends ultimate reality to these figures in the metro is the fact that they are figures of movement.