assign :: Parser Statement
assign = do name <- identifier
            reservedOp ":="
            value <- arithmeticExpression
            return (Assign name value)
assign :: Parser Statement
assign = do name <- identifier
            reservedOp ":="
            value <- arithmeticExpression
            return (Assign name value)
assign = do name <- identifier
            reservedOp ":="
            value <- arithmeticExpression
            return (Assign name value)