public void addParameter(String param) {
        verifyThat(! parameters.contains(param), "cannot have parameter", param, "twice");
        parameters.add(param);
    }
This class holds the extracted information after parsing a
"function" block in a rank-profile, using simple data structures as
far as possible.  Do not put advanced logic here!
@author arnej27959
config-model/src/main/java/com/yahoo/schema/parser/ParsedRankFunction.java