(# fs fs source-file-name 
      ft target-file-name .source .target -
    (+FS_REQUEST
     +FS_OPEN_IF_EXISTS
     +FS_RETURN_ON_ERROR
     +FS_READ .fs) |
    (+FS_REQUEST
     +FS_REPLACE_IF_EXISTS
     +FS_RETURN_ON_ERROR
     +FS_WRITE .ft)
)
('& [source] && [target] >
    (# char-in source .text
     # char-out target text)
    ...
)
