The function takes a string as input, keeps the first character unchanged, and performs a cyclic character substitution for the remaining characters in the input string. The function leaves digits and special characters unchanged.

[CODE]:
```python
def f(s):
    if len(s) == 0:
        return s
    else:
        result = [s[0]]
        for char in s[1:]:
            if 'a' <= char <= 'y':
                result.append(chr(ord(char) + 1))
            elif 'A' <= char <= 'Y':
                result.append(chr(ord(char) + 1))
            elif char == 'z':
                result.append('a')
            elif char == 'Z':
                result.append('A')
            else:
                result.append(char)
        return ''.join(result)
```

Please note that this code is an approximation of the behavior of the original function and might not be exactly the same.