def atan(x):
 np = import_module('numpy')
 if isinstance(x, (int, float)):
  return interval(np.arctan(x))
 elif isinstance(x, interval):
  start = np.arctan(x.start)
  end = np.arctan(x.end)
  return interval(start, end, is_valid=x.is_valid)
 else:
  raise NotImplementedError

def atan ( x ) : np = import_module ( 'numpy' ) if isinstance ( x , ( int , float ) ) : return interval ( np . arctan ( x ) ) elif isinstance ( x , interval ) : start = np . arctan ( x . start ) end = np . arctan ( x . end ) return interval ( start , end , is_valid = x . is_valid ) else : raise NotImplementedError
evaluates the tan inverse of an interval .