docs: def define(self, *names, **kwargs):
          """Define a variable in the problem.
  
          Variables must be defined before they can be accessed by var() or
          set(). This function takes keyword arguments lower and upper to define
          the bounds of the variable (default: -inf to inf). The keyword argument
          types can be used to select the type of the variable (Continuous
          (default), Binary or Integer). Setting any variables different than
          Continuous will turn the problem into an MILP problem. Raises
          ValueError if a name is already defined.
          """
          names = tuple(names)
          for name in names:
              if name in self._variables:
                  raise ValueError('Variable already defined: {!r}'.format(name))
  
          lower = kwargs.get('lower', None)
          upper = kwargs.get('upper', None)
          vartype = kwargs.get('types', None)
  
          # Repeat values if a scalar is given
          if lower is None or isinstance(lower, numbers.Number):
              lower = repeat(lower, len(names))
          if upper is None or isinstance(upper, numbers.Number):
              upper = repeat(upper, len(names))
          if vartype is None or vartype in (
                  VariableType.Continuous, VariableType.Binary,
                  VariableType.Integer):
              vartype = repeat(vartype, len(names))
  
          lp_names = tuple(next(self._var_names) for name in names)
  
          # Assign default values
          lower = (-cp.infinity if value is None or value == -_INF
                   else float(value) for value in lower)
          upper = (cp.infinity if value is None or value == _INF
                   else float(value) for value in upper)
          vartype = tuple(VariableType.Continuous if value is None else value
                          for value in vartype)
  
          args = {'lb': tuple(lower), 'ub': tuple(upper)}
          if any(value != VariableType.Continuous for value in vartype):
              # Set types only if some are integer (otherwise Cplex will change
              # the solver to MILP).
              args['types'] = tuple(Problem.VARTYPE_MAP[t] for t in vartype)
  
          self._variables.update(zip(names, lp_names))
          self._cp.variables.add(**args)