By automatically locating fiducial markers, the camera automates board alignment. Vision systems in the stencil printing machines use global fiducial marks for aligning the PWB. An ARTag is a fiducial marker system to support augmented reality. Commonly used fiducial markers are fluorescently labelled monomers of bio-polymers. Tattoos have been used as fiducial markers as an aid in guiding radiotherapy. Some methods assume objects with known geometry (or fiducial markers) are present in the scene. The general approach of fiducial inference was proposed by Ronald Fisher. Notice that the fiducial distribution is uniquely defined when a single sufficient statistic exists. Other current work in fiducial inference is ongoing under the name of confidence distributions. Lindley showed that fiducial probability lacked additivity, and so was not a probability measure. Fisher required the existence of a sufficient statistic for the fiducial method to apply. Fisher sketched "proofs" of results using fiducial probability. Further discussion of fiducial inference is given by Kendall & Stuart (1973). Fisher admitted that "fiducial inference" had problems. Most of the tangibles are flat, with one fiducial on the underside.