1. A method of formal analysis, comprising: classifying a first primary output of a first data-flow graph to determine a first data-flow graph class; classifying a second primary output of a second data-flow graph to determine a second data-flow graph class; generating a combining structure by combining the first data-flow graph and the second data-flow graph, wherein the combining structure is a data-flow graph; selecting a suitable type for the combining structure, for coupling the first and second primary outputs, based upon the first data-flow graph class and the second data-flow graph class, wherein selecting the suitable type of combining structure involves selecting a pipelined combining structure when the first primary output is classified as pipelined and the second primary output is classified as combinational; and applying a formal analysis process to the first data-flow graph, the second data-flow graph and the combining structure, wherein performing an equivalency test on a high-level model and register transfer level (RTL) model, and wherein the high-level model is converted into the first data-flow graph, and wherein the RTL model is converted into the second data-flow graph.