A toy example of the filtering using correlation. Circles represent various classes and thick line shows correlation between classes and dotted line an weak correlation. Classes have no correlation, when they lack a connecting line. Figure shows that if we have already class A among selected signal classes the first filtering discards classes B and E. Note that second filtering discards class C as we have a path of strong correlations through class B to it. Class D is left unfiltered. Positioning of circles in the figure is unrelated to the GO structure.