docs: def find_common_dtype(*args):
      '''Returns common dtype of numpy and scipy objects.
  
      Recognizes ndarray, spmatrix and LinearOperator. All other objects are
      ignored (most notably None).'''
      dtypes = []
      for arg in args:
          if type(arg) is numpy.ndarray or  \
                  isspmatrix(arg) or \
                  isinstance(arg, LinearOperator):
              if hasattr(arg, 'dtype'):
                  dtypes.append(arg.dtype)
              else:
                  warnings.warn('object %s does not have a dtype.'
                                % arg.__repr__)
      return numpy.find_common_type(dtypes, [])