A reflective mask blank for EUV lithography having an absorbing layer which has a low reflectivity with respect to wavelength regions of EUV light and pattern inspection light, and which is easily controllable to obtain desired film composition and film thickness. The thrombus filter includes a body portion and a plurality of struts, each strut having a joined end and a free end. User interfaces, designed to assist in the visualization of a particular form of knowledge, allow users to describe the complex system with simple interactive elements. The user commands may be movements made at a user interface that correspond to movements of the medical instrument.