Constructs a new AttributePredicate, given an attribute name
and a target literal (with which to test equality).
if (attribute == null)
throw new IllegalArgumentException("non-null attribute needed");
if (attribute.indexOf(":") != -1)
throw new IllegalArgumentException(
"namespace-qualified attribute names are not currently " +
"supported");
this.attribute = attribute;
if (target == null)
throw new IllegalArgumentException("non-null target needed");
// strip quotation marks from target
this.target = target.substring(1, target.length() - 1);
