def strip_empty_lines(s):
    """
    Removes all empty lines from the multi-line string `s`.
    
    Examples
    --------
    
    >>> multiline = '''A string with
    ... 
    ... an empty line.'''
    >>> print(strip_empty_lines(multiline))
    A string with
    an empty line.
    """
    return '\n'.join(line for line in s.split('\n') if line.strip())