private String getWhenClausePredicate(ASTNode whenClause) {
    if (!(whenClause.getType() == HiveParser.TOK_MATCHED || whenClause.getType() == HiveParser.TOK_NOT_MATCHED)) {
      throw raiseWrongType("Expected TOK_MATCHED|TOK_NOT_MATCHED", whenClause);
    }
    if (whenClause.getChildCount() == 2) {
      return getMatchedText((ASTNode)whenClause.getChild(1));
    }
    return null;
  }
Returns the <boolean predicate> as in WHEN MATCHED AND <boolean predicate> THEN...
@return may be null
ql/src/java/org/apache/hadoop/hive/ql/parse/MergeSemanticAnalyzer.java