#
  # NOTE: This procedure verifies the combination of language and version
  #       specified by the caller.  The language argument must be one of the
  #       literal strings "eagle", "tcl", or "client".  The version argument
  #       must be one of the literal strings "8.4", "8.5", or "8.6" when the
  #       language is "tcl" -OR- the literal string "1.0" when the language
  #       is "eagle".  When the language is "client", the version match the
  #       major and minor portions of "1.0" and any remaining portions must
  #       is either "eagle" or "client".  The varName argument is the name
  #       of a scalar variable in the context of the immediate caller that
  #       will receive a boolean value indicating if the specified language
  #       is actually a reference to the package downloader client itself.
  #       be numeric.  The varName argument is the name of a scalar variable
  #       in the context of the immediate caller that will receive a boolean
  #       value indicating if the specified language is actually a reference
  #       to the package downloader client itself.
  #
  # <internal>
  proc verifyLanguageAndVersion { language version varName } {
    if {[string length $varName] > 0} then {
      upvar 1 $varName isClient
    }

    set isClient false

    if {[string length $language] == 0 || $language eq "eagle"} then {
      if {$version ne "1.0"} then {
        error "unsupported Eagle version"
      }
    } elseif {$language eq "tcl"} then {
      if {$version ne "8.4" && $version ne "8.5" && $version ne "8.6"} then {
        error "unsupported Tcl version"
      }
    } elseif {$language eq "client"} then {
      if {$version ne "1.0"} then {
      if {![regexp -- {^1\.0(?:\.\d+)+$} $version]} then {
        error "unsupported client version"
      }

      set isClient true
    } else {
      error "unsupported language"
    }