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
concatenatio
n
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.