parallel as possible (similar to GNU Make's `-j' switch).  This is
  useful on SMP systems, but it is especially useful for doing builds
  on multiple machines.  The idea is that a large derivation is
  initiated on one master machine, which then distributes
  sub-derivations to any number of slave machines.  This should not
  happen synchronously or in lock-step, so the master must be capable
  of dealing with multiple parallel build jobs.  We now have the
  infrastructure to support this.

  TODO: substitutes are currently broken.