def remove(s1,s2):
    """
    Returns a copy of s, with all characters in s2 removed.
    
    Examples:
        remove('abc','ab') returns 'c'
        remove('abc','xy') returns 'abc'
        remove('hello world','ol') returns 'he wrd'
    
    Parameter s1: the string to copy
    Precondition: s1 is a string
    
    Parameter s2: the characters to remove
    Precondition: s2 is a string
    """
    result = ''
    for x in s1:
        if not x in s2:
            result = result + x
    return result