(lc=
  counts c
.     fil$(!arg,r)                        {open file for reading}
    & 0:?counts
    &   whl
      ' ( fil$:?c                         {read a byte}
        &     ( !c:(~<A:~>Z|~<a:~>z)
              | 0
              )
            + !counts
          : ?counts                       {simply add any found letter to the sum}
        )
    & fil$(,SET,-1)                       {close the file by seeking to impossible file position.}
  | !counts                               {return the sum}
);

lc$"valid.bra"                            {example: count letters in Bracmat's validation suite.}