Represents the name of a formal argument
defined in a template:
group test;
test(a,b) : "$a$ $b$"
t() : "blort"
Each template has a set of these formal arguments or uses
a placeholder object: UNKNOWN (indicating that no arguments
were specified such as when a template is loaded from a file.st).
Note: originally, I tracked cardinality as well as the name of an
attribute.