def is_base_pair(s1, s2):
    """ (str, str) -> bool
    
    Precondition: s1 and s2 both contain a single character from 'A', 'T', 'C'
    or 'G'.
    
    Return True iff s1 and s2 form a base pair.
    
    >>> is_base_pair('A','T')
    True
    >>> is_base_pair('G','T')
    False
    """
    
    cond1 = (s1 == 'A' and s2 == 'T')
    cond2 = (s1 == 'T' and s2 == 'A')
    cond3 = (s1 == 'G' and s2 == 'C')
    cond4 = (s1 == 'C' and s2 == 'G')
    
    if cond1 or cond2 or cond3 or cond4:
        return True
    else:
        return False