# Task
 Given a rectangular `matrix` and integers `a` and `b`, consider the union of the ath row and the bth (both 0-based) column of the `matrix`. Return sum of all elements of that union.
# Example
 For
```
matrix = [[1, 1, 1, 1], 
          [2, 2, 2, 2], 
          [3, 3, 3, 3]]
a = 1 and b = 3 ```
the output should be `12`.
 Here `(2 + 2 + 2 + 2) + (1 + 3) = 12`.
# Input/Output
 - `[input]` 2D integer array `matrix`
    2-dimensional array of integers representing a rectangular matrix.
    Constraints: `1 ≤ matrix.length ≤ 5, 1 ≤ matrix[0].length ≤ 5, 1 ≤ matrix[i][j] ≤ 100.`
 - `[input]` integer `a`
  A non-negative integer less than the number of matrix rows.
   Constraints: `0 ≤ a < matrix.length.`
 - `[input]` integer `b`
   A non-negative integer less than the number of matrix columns.
   Constraints: `0 ≤ b < matrix[i].length. `
 - `[output]` an integer
def crossing_sum(matrix, row, col):
    return sum(matrix[row]) + sum(i[col] for i in matrix[:row] + matrix[row + 1:])
def crossing_sum(matrix, row, col):
    return sum(matrix[row]) + sum(i[col] for i in matrix[:row] + matrix[row + 1:])