Given a 7x7 grid of letters {a..g}, some cells are pre-filled. Fill the rest so that:
1) Each row has exactly one of each letter {a..g}.
2) Each column has exactly one of each letter {a..g}.
3) All cells on the minor diagonal (top-right to bottom-left) contain the same letter.

Here is the puzzle (each row on its own line, cells separated by commas, empty cells blank):

,,c,,e,f,
,c,b,,f,,a
,b,e,,d,a,
b,,f,d,a,g,c
e,,d,a,,c,b
f,d,a,g,,,
,a,g,,,e,

Return the answer with the format:
<<<
row1
row2
...  (7 rows total)
>>>

where each row has 7 letters separated by commas.
