This 'calc' example demonstrates a few special abilities of the styx library:

You can (but need not) use the styx library together with C++.

The parser is capable of reading sources from files and strings. 
The 'calc' example reads from stdin. 
The source 'pattern_file.txt' demonstrates the parsing from a file.

Use the shell script 'mx' to create the executable. 
Don't forget 
1) to check the setting of the environment variable STYXDIR in 'mx'
2) to add $STYXDIR/lib to your library path.