<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def eq_add(self, expr, var, intf=False):
 """ Add an equation to this model. An equation is associated with the addresses of a variable. The number of equations must equal that of variables. Stored to ``self._equations`` is a tuple of ``(expr, var, intf, ty)`` where ``ty`` is in ('f', 'g') :param str expr: equation expression :param str var: variable name to be associated with :param bool intf: if this equation is added to an interface equation, namely, an equation outside this model :return: None """


        # determine the type of the equation, ('f', 'g') based on var

        # We assume that all differential equations are only modifiable by
        #   the model itself
        # Only interface to algebraic varbailes of external models

        ty = ''
        if var in self._algebs:
            ty = 'g'
        elif var in self._states:
            ty = 'f'
        else:
            for _, item in self._ac:
                if var in item:
                    ty = 'g'
                    if intf is False:
                        intf = True
            for _, item in self._dc:
                if var == item:
                    ty = 'g'
                    if intf is False:
                        intf = True
        if ty == '':
            self.log(
                'Equation associated with interface variable {var} '
                'assumed as algeb'.
                format(var=var),
                DEBUG)
            ty = 'g'

        self._equations.append((expr, var, intf, ty))