///public func <<<T>(array: [T], offset: Int) 
//==mark==public func <<<T>(array: [T], offset: Int) 
public func <<<T>(array: [T], offset: Int) -> [T] { return array.shiftedLeft(by: offset)  }

sed '/public func/ {
                    h
                    s/^[[:blank:]]*//\/\/\//
                    s/\([^)]*)\).*/\1/p
                    s/^\/\/\//\/\/==mark==/p
                    g
               }' /path/to/file

sed '/public func/ {                          # for every line with "pub..." do:
               h                              # copy line to hold space
               s/^[[:blank:]]*//\/\/\//       # replace leading blanks (if any) with three slashes
               s/\([^)]*)\).*/\1/p            # remove everything after first ")" and print the result
               s/^\/\/\//\/\/==mark==/p       # replace leading "///" with "//==mark==" and print again
               g                              # copy hold space (original line) back to pattern space
     }'