from typing import TextIO

def write_ascii_triangle(outfile: TextIO, block: str, sidelength: int) -> None:
    """Write an ascii isosceles right triangle using block that is sidelength
    characters wide and high to outfile. The right angle should be in the
    upper-left corner. 

    Precondition: len(block) == 1

    For example, given block="@" and sidelength=4, the
    following should be written to the file:
    
    @@@@
    @@@
    @@
    @
    """