code: func Eq(lhs, rhs Expression) BoolExpression {
  	lit, ok := rhs.(*literalExpression)
  	if ok && sqltypes.Value(lit.value).IsNull() {
  		return newBoolExpression(lhs, rhs, []byte(" IS "))
  	}
  	return newBoolExpression(lhs, rhs, []byte("="))
  }
docs: // Returns a representation of "a=b"