Represents the name of a formal argument
defined in a template:
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. I'm leaving the code here as I suspect something may come
of it later. Currently, though, cardinality is not used.
When template arguments are not available such as when the user
uses "new StringTemplate(...)", then the list of formal arguments
must be distinguished from the case where a template can specify
args and there just aren't any such as the t() template above.
If they specified name="value", store the template here
default : return "unknown";