The name of the parameter. If this attribute is missing, the value is interpreted as a , separated list of name=value pairs.
The value of the parameter. Is this attribute, and 'referid', is missing, then the body of the param tag is used.
The value of the parameter, as referred from the variable context.
The `vartype' of the parameter. In principle url-parameters should always be string. When
using this attribute, the value is first cast to this type. You could e.g. set it to
`integer' if you want to ensure the variable is parseable as an integer.