Object concatenation

Normally a string "abcb" represents a sequence of objects 'a','b','c' such that:

'b' starts at the moment 'a' stops; 'c' starts at the moment 'b' stops; 'b' starts again at the moment 'c' stops.

What if we want 'b' to continue while 'c' is on? In other words, we wish to represent:

Fig.5 Using the concatenation symbol
This can be done using the concatenation symbol '&'. The proper representation is

a b& c &b

Beware 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,

a b& c& d c &c b &b

may be interpreted as:

Fig.6 Using the concatenation symbol (more complex)

Object concatenation 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.