changed:
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Possible classes to describe the function of PD externals:


    * Control

        * Flow (bang, trigger, spigot ...)

        * Math (float, +-*/ == <= ...) 

        * Lists (pack, unpack, glue ...)

        * Symbols (makesymbol, makefilename ...)

        * MIDI (notein, ctlin ...)

        * Data-structures (...)
 
        * Database (pool, qlist? textfile? ...)

    * Signal

        * Flow (dac~, adc~, sig~. )

        * Generators (osc~, cos~, phasor~ ...)

        * Tables (tabread, tabwrite, soundfiler...)

        * Filters (env~, vcf~ ...)

        * FFT & Audio Math (fft~, clip~ ...)

    * Graphics

        * Pixel Inputs (pdp_qt, pix_video)

        * Pixel Filters (pdp_convolve, pix_negative)

        * Pixel Outputs (pdp_xv, pix_write, pix_draw)

        * GemChains (pdp2gem, translate, rotate ...)

        * Lighting (light, world_light)

        * Particle (part_*)

        * Vertex (vertex_*)

This first effort is multiple-level since I don't think a flat list of functional keywords will have a valuable granularity. No class list is perfect and things belong to any catagory only to some degree. Nevertheless such a structure breaks up the massive list of PD externals in a meaningful way to novice users. Please contribute!