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11.12.9 Cannot bind external port to non-external port

There is a restriction in the binding of external ports: when an external requires port of a system Component is bound, the other side of the binding must be an external requires port also (this is only possible when that is a port of a sub Component). In the example below some errors are reported. 

interface i {
  in void e ();
  behavior {
    on e: {}
  }
}

component c1 {
  provides i p;
  requires external i r1;
  requires external i r2;
  behavior {
    on p.e (): {}
  }
}

component c2 {
  provides i p;
  behavior {
    on p.e (): {}
  }
}

component s1 {
  provides i p;
  requires i r;
  system {
    c1 ci1;
    c2 ci2;
    p <=> ci1.p;
    ci1.r1 <=> r;
    ci1.r2 <=> ci2.p;
  }
}

component s2 {
  provides i p1;
  provides i p2;
  requires external i r1;
  requires external i r2;
  system {
    s1 si1;
    p1 <=> si1.p;
    p2 <=> r2;
    r1 <=> si1.r;
  }
}

This results in the following error message: 

binding-mismatch-external.dzn:45:5: error: cannot bind non-external port
    `r' to external port `r1'
binding-mismatch-external.dzn:26:3: info: port `r' defined here
binding-mismatch-external.dzn:39:3: info: port `r1' defined here