A chess board is normally played with 16 pawns and 16 other pieces, for this kata a variant will be played with only the pawns.  All other pieces will not be on the board.  
For information on how pawns move, refer [here](http://www.chesscorner.com/tutorial/basic/pawn/pawn.htm)
Write a function that can turn a list of pawn moves into a visual representation of the resulting board.  
A chess move will be represented by a string,
```
"c3"
```
This move represents a pawn moving to `c3`.  If it was white to move, the move would represent a pawn from `c2` moving to `c3`.  If it was black to move, a pawn would move from `c4` to `c3`, because black moves in the other direction.  
The first move in the list and every other move will be for white's pieces.
The letter represents the column, while the number represents the row of the square where the piece is moving  
Captures are represented differently from normal moves:
```
"bxc3"
```
represents a pawn on the column represented by 'b' (the second column) capturing a pawn on `c3`.
For the sake of this kata a chess board will be represented by a list like this one:  
```
[[".",".",".",".",".",".",".","."],
 ["p","p","p","p","p","p","p","p"],
 [".",".",".",".",".",".",".","."],
 [".",".",".",".",".",".",".","."],
 [".",".",".",".",".",".",".","."],
 [".",".",".",".",".",".",".","."],
 ["P","P","P","P","P","P","P","P"],
 [".",".",".",".",".",".",".","."]]
```
Here is an example of the board with the squares labeled:
```
[["a8","b8","c8","d8","e8","f8","g8","h8"],
 ["a7","b7","c7","d7","e7","f7","g7","h7"],
 ["a6","b6","c6","d6","e6","f6","g6","h6"],
 ["a5","b5","c5","d5","e5","f5","g5","h5"],
 ["a4","b4","c4","d4","e4","f4","g4","h4"],
 ["a3","b3","c3","d3","e3","f3","g3","h3"],
 ["a2","b2","c2","d2","e2","f2","g2","h2"],
 ["a1","b1","c1","d1","e1","f1","g1","h1"]]
```
White pawns are represented by capital `'P'` while black pawns are lowercase `'p'`.  
A few examples
```
If the list/array of moves is: ["c3"]
>>>
[[".",".",".",".",".",".",".","."],
 ["p","p","p","p","p","p","p","p"],
 [".",".",".",".",".",".",".","."],
 [".",".",".",".",".",".",".","."],
 [".",".",".",".",".",".",".","."],
 [".",".","P",".",".",".",".","."],
 ["P","P",".","P","P","P","P","P"],
 [".",".",".",".",".",".",".","."]]
```
add a few more moves,
```
If the list/array of moves is: ["d4", "d5", "f3", "c6", "f4"]
>>>
[[".",".",".",".",".",".",".","."],
 ["p","p",".",".","p","p","p","p"],
 [".",".","p",".",".",".",".","."],
 [".",".",".","p",".",".",".","."],
 [".",".",".","P",".","P",".","."],
 [".",".",".",".",".",".",".","."],
 ["P","P","P",".","P",".","P","P"],
 [".",".",".",".",".",".",".","."]]
```
now to add a capture...
```
If the list/array of moves is: ["d4", "d5", "f3", "c6", "f4", "c5", "dxc5"]
>>>
[[".",".",".",".",".",".",".","."],
 ["p","p",".",".","p","p","p","p"],
 [".",".",".",".",".",".",".","."],
 [".",".","P","p",".",".",".","."],
 [".",".",".",".",".","P",".","."],
 [".",".",".",".",".",".",".","."],
 ["P","P","P",".","P",".","P","P"],
 [".",".",".",".",".",".",".","."]]
```
If an invalid move (a move is added that no pawn could perform, a capture where there is no piece, a move to a square where there is already a piece, etc.) is found in the list of moves, return '(move) is invalid'.
```python
If the list/array of moves is: ["e6"]
>>>
"e6 is invalid"
```
```python
If the list/array of moves is: ["e4", "d5", "exf5"]
>>>
"exf5 is invalid"
```
The list passed to `pawn_move_tracker / PawnMoveTracker.movePawns` will always be a list of strings in the form (regex pattern): `[a-h][1-8]` or `[a-h]x[a-h][1-8]`.  
Notes:
* In the case of a capture, the first lowercase letter will always be adjacent to the second in the alphabet, a move like `axc5` will never be passed.
* A pawn can move two spaces on its first move
* There are no cases with the 'en-passant' rule.
LETTERS        = 'abcdefgh'
NUMBERS        = '87654321'
W, B = WB      = 'Pp'
EMPTY, CAPTURE = '.x'
WHITEHOME      = '12'
BLACKHOME      = '87'
JUMP           = '54'

def pawn_move_tracker(moves):
    board = {letter + number :
             B if number == BLACKHOME[1] else W if number == WHITEHOME[1] else EMPTY
             for letter in LETTERS for number in NUMBERS}
    whitemove = True
    for move in moves:
        target    = move[-2:]
        mover     = move[0] + str(int(move[-1]) + 1 - whitemove * 2)
        if  move[-1] in JUMP[whitemove] and board[mover] == EMPTY:
            mover = move[0] + str(int(move[-1]) + 2 - whitemove * 4)
        if (move[-1] in (BLACKHOME, WHITEHOME)[whitemove]           or
            board[target] != (EMPTY, WB[whitemove])[move[1] == CAPTURE] or
            board[mover]  != WB[not whitemove]):
            return "{} is invalid".format(move)
        whitemove = not whitemove
        board[mover]  = EMPTY
        board[target] = WB[whitemove]
    return [[board[letter + number] for letter in LETTERS] for number in NUMBERS]

LETTERS        = 'abcdefgh'
NUMBERS        = '87654321'
W, B = WB      = 'Pp'
EMPTY, CAPTURE = '.x'
WHITEHOME      = '12'
BLACKHOME      = '87'
JUMP           = '54'

def pawn_move_tracker(moves):
    board = {letter + number :
             B if number == BLACKHOME[1] else W if number == WHITEHOME[1] else EMPTY
             for letter in LETTERS for number in NUMBERS}
    whitemove = True
    for move in moves:
        target    = move[-2:]
        mover     = move[0] + str(int(move[-1]) + 1 - whitemove * 2)
        if  move[-1] in JUMP[whitemove] and board[mover] == EMPTY:
            mover = move[0] + str(int(move[-1]) + 2 - whitemove * 4)
        if (move[-1] in (BLACKHOME, WHITEHOME)[whitemove]           or
            board[target] != (EMPTY, WB[whitemove])[move[1] == CAPTURE] or
            board[mover]  != WB[not whitemove]):
            return "{} is invalid".format(move)
        whitemove = not whitemove
        board[mover]  = EMPTY
        board[target] = WB[whitemove]
    return [[board[letter + number] for letter in LETTERS] for number in NUMBERS]
