code: public String getSingle(String attribute) {
  	NameOpValue nv = rslTree.getParam(attribute);
  	if (nv == null || nv.getOperator() != NameOpValue.EQ) return null;
  	Object obj = nv.getFirstValue();
  	if (obj != null && obj instanceof Value) {
  	    return ((Value)obj).getCompleteValue();
  	} else {
  	    return null;
  	}
      }
docs: Returns a string value of the specified attribute.
  If the attribute contains multiple values the
  first one is returned.
  
  @param attribute the rsl attribute to return the value of.
  @return value of the relation. Null is returned if there is
  no such attribute of the attribute/value relation is
  not an equality relation.