public ExpressionNode parseRelExpression(ExpressionNode shift)
Parses a RelExpression
relExpression:
     shiftExpression
   | relExpression relOperator shiftExpression
   ;
relOperator:
     '<'
   | '<='
   | '>'
   | '>='
   | '!<>='
   | '!<>'
   | '<>'
   | '<>='
   | '!>'
   | '!>='
   | '!<'
   | '!<='
   ;