#       errors are raised if any problems are found.  The return value is
  #       undefined.
  #
  # <public>
  proc openPgpMustBeInstalled {} {
    variable openPgpInstalledCommand
    variable openPgpInstalledPattern

    set message [string trim {
      Cannot use OpenPGP: it does not appear to be installed.

      GNU Privacy Guard (GPG) may be downloaded from "https://www.gnupg.org/"
      and then installed.  Signed binaries for Windows may be available from
      "https://www.gpg4win.org/".

      Alternatively, it may be possible to install GNU Privacy Guard (GPG) via
      the package management subsystem included with your operating system.
    }]

    if {[isEagle]} then {
      if {[catch {
        eval exec -success Success $openPgpInstalledCommand
      } result]} then {
        error $message
      }
    } else {
      if {[catch {
        eval exec $openPgpInstalledCommand
      } result]} then {
        error $message
      }
    }

    if {![info exists result] || \
        ![regexp -- $openPgpInstalledPattern $result]} then {
      error "cannot use OpenPGP: unknown or unsupported version"
    }