setmerge.icn: Program to combine sets of text items

Setmerge combines sets of items according to the specified operators.
Sets are read from files, one entry per line.  Operation is from left
to right without any precedence rules.  After all operations are
complete the resulting set is sorted and written to standard output.

Usage:  setmerge file [[op] file]...

Operations:
     +  add contents to set
     -  subtract contents from set
     *  intersect contents with set

Note that operators must be separate command options, and that some
shells my require some of them to be quoted.

Example 1: combine files, sorting and eliminating duplicates:

     setmerge file1 + file2 + file3 + file4

Example 2: print lines common to three files

     setmerge file1 '*' file2 '*' file3

Example 3: print lines in file1 or file2 but not in file3

     setmerge file1 + file2 - file3