The Bottom of the Matrix

This challenge concerns square matrices (same number of rows and columns) as the below example illustrates:

[
  [1, 2, 3],
  [4, 5, 6],
  [7, 8, 9]
]

The entries in the diagonal line from the top left to the bottom right form the main diagonal of the matrix. In this case, 1,5,9 form the main diagonal.

Write a function that returns the matrix obtained by replacing the entries above the main diagonal with 0s.

For example, for the matrix above you should return:

[
  [1, 0, 0],
  [4, 5, 0],
  [7, 8, 9]
]

Examples

lowerTriang([
  [1, 2, 3],
  [4, 5, 6],
  [7, 8, 9]
]) ➞ [
  [1, 0, 0],
  [4, 5, 0],
  [7, 8, 9]
]

lowerTriang([
  [5, 7],
  [7, 9]
]) ➞ [
  [5, 0],
  [7, 9]
]

lowerTriang([
  [1, 8, 8, 1],
  [2, 7, 7, 2],
  [3, 6, 6, 3],
  [4, 5, 5, 4]
]) ➞ [
  [1, 0, 0, 0],
  [2, 7, 0, 0],
  [3, 6, 6, 0],
  [4, 5, 5, 4]
]

Notes

• As in the examples, the size of the matrices will vary (but they will always be square).
• In Linear Algebra, matrices with 0s above the diagonal are called lower triangular matrices.