To convert a sieve script into equivalent Scheme program, executable
by guimb, run:

  guile -s sieve.scm --file <sieve-script-name> --output <output-file-name>

To compile and execute a sieve script upon a mailbox, run:

  guimb -f sieve.scm -{ --file <sieve-script-name> -} --mailbox ~/mbox