protected ASTNode findLHSofAssignment(ASTNode assignment) {
    assert assignment.getToken().getType() == HiveParser.EQUAL :
      "Expected set assignments to use equals operator but found " + assignment.getName();
    ASTNode tableOrColTok = (ASTNode)assignment.getChildren().get(0);
    assert tableOrColTok.getToken().getType() == HiveParser.TOK_TABLE_OR_COL :
      "Expected left side of assignment to be table or column";
    ASTNode colName = (ASTNode)tableOrColTok.getChildren().get(0);
    assert colName.getToken().getType() == HiveParser.Identifier :
      "Expected column name";
    return colName;
  }
Assert that we are not asked to update a bucketing column or partition column.
@param colName it's the A in "SET A = B"
ql/src/java/org/apache/hadoop/hive/ql/parse/RewriteSemanticAnalyzer.java