Based on the provided input-output pairs, it seems that the function `f(x)` performs the following operations:

1. If the input string contains alphabetic characters, it replaces the first vowel in each word of the string with 'd'. 
2. If the input string contains only numeric characters, it returns the input string as is.

Here is an approximation of the behavior of the function in Python code:

[DESCRIPTION]:
The function `f(x)` replaces the first vowel in each word of an input string with 'd', but leaves numeric strings unchanged.

[CODE]:
```python
def f(x):
    vowels = ['a', 'e', 'i', 'o', 'u']
    words = x.split()
    
    if all(char.isdigit() for char in x):
        return x
    else:
        for i in range(len(words)):
            for vowel in vowels:
                if vowel in words[i]:
                    words[i] = words[i].replace(vowel, 'd', 1)
                    break
                    
        return ' '.join(words)
```