ProofContext.read_const: allow for type constraint (for fixed variable);
added proof command 'write' to introduce concrete syntax within a proof body;
/***************************************************************************
  Title:      GraphBrowser/DummyVertex.java
  Author:     Stefan Berghofer, TU Muenchen
  This class represents a dummy vertex, which is used to simplify the
  layout algorithm.
***************************************************************************/
package GraphBrowser;
import java.awt.*;
class DummyVertex extends Vertex {
	public boolean isDummy() {return true;}
	public Object clone() {
		Vertex ve=new DummyVertex();
		ve.setX(getX());ve.setY(getY());
		return ve;
	}
	public int leftX() { return getX(); }
	public int rightX() { return getX(); }
	public void draw(Graphics g) {}
}