  Cubic bipartite graphs with girth at least 6 correspond to symmetric
combinatorial (v_3) configurations. In 1887 V. Martinetti described a simple
reduction method which enables one to reduce each combinatorial (v_3)
configuration to one from the infinite set of so-called irreducible
configurations. The aim of this paper is to show that a slightly extended set
of reductions enables one to reduce each combinatorial (v_3) configuration
either to the Fano configuration or to the Pappus configuration.
