a string "abcb" represents a sequence of objects 'a','b','c' such that:
starts at the moment 'a' stops; 'c' starts at the moment 'b' stops; 'b' starts
again at the moment 'c' stops.
if we want 'b' to continue while 'c' is on? In other words, we wish to
Using the concatenation symbol
can be done using the
'&'. The proper representation is
b& c &b
that spaces are meaningful in this representation: you can't write "b&c".
More complex structures can be defined in a similar way. For instance,
b& c& d c &c b &b
be interpreted as:
Using the concatenation symbol (more complex)
is used for representing objects which belong to several substructures, i.e.
that are not structured as a tree hierarchy. See for instance the grammar
generating Ames' example in §4.10.