reserve S for IncStruct;
reserve A,B,C,D for POINT of S;
reserve L for LINE of S;
reserve P for PLANE of S;
reserve F,G for Subset of the Points of S;

theorem
  G c= F & F is planar implies G is planar
proof
  assume that
A1: G c= F and
A2: F is planar;
  consider P such that
A3: F on P by A2;
  take P;
  let A be POINT of S;
  assume A in G;
  hence thesis by A1,A3;
end;
