Insert character at a fixed position of lines

[Francesco Pietra]

How to insert letter "A" on each line (of a very long list of lines)
at position 22, i.e., one space after "LEU", leaving all other
characters at the same position as in the original example:


ATOM      1  N   LEU     1     146.615  40.494 103.776  1.00 73.04       1SG   2

In all lines"ATOM" is constant as to both position and string, while
"LEU" is constant as to position only, i.e., "LEU" may be replaced by
three different uppercase letters. Therefore, the most direct
indication would be position 22.

Should the script introduce blank lines, no problem. That I know how
to correct with a subsequent script.

Thanks
chiendarret