for pre in *.pre.txt
  diff -urN \
    (sed -E "s/ '--(swift|llvm)-cmake-options=[^']+'//g" $pre | psub)
    (sed -E "s/ '--(swift|llvm)-cmake-options=[^']+'//g" (string replace -r '\.pre\.' '.output.' $pre) | psub)
  or read
end