<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:

def common_prefix(s1, s2):
    "Return prefix common of 2 strings"
    if not s1 or not s2:
        return ""
    k = 0
    while s1[k] == s2[k]:
        k = k + 1
        if k >= len(s1) or k >= len(s2):
            return s1[0:k]
    return s1[0:k]