A <pattern abstract="true"> is a template whose rule queries carry $name
references; a <pattern is-a="…"> with <param> children instantiates it,
substituting each parameter into the template's context, test, <let>
value, and <value-of select=> before the XPath is compiled. The
substitution is a single left-to-right pass, so a substituted value is
never rescanned and a short name never captures a longer one; $name
references that are not parameters stay as written. The template itself
contributes no rules.
