if not modules then modules = { } end modules ['mlib-msy'] = {
    version   = 1.001,
    comment   = "companion to mlib-lmt.mkxl",
    author    = "Hans Hagen & Mikael Sundqvist",
    copyright = "PRAGMA ADE / ConTeXt Development Team",
    license   = "see context related readme files",
}

-- This is yet another playful exploration by Mikael Sundqvist and Hans Hagen. We
-- started from just macros doing turtle graphics. From that we moved to wrappers
-- and finally to a simple \LUA\ parser so that we could more easilly enter
-- definitions. Of course we can do all calculations and path construction in \LUA,
-- but we like to test and apply the relative path builder code. In addition it is a
-- playground for the new \type {lastxy} primitive. Maybe we should test this hiding
-- mechanism a bit more for performance.

local P, R, S, C, Cs, lpegmatch = lpeg.P, lpeg.R, lpeg.S, lpeg.C, lpeg.Cs, lpeg.match
local find, formatters = string.find, string.formatters
local concat = table.concat

local getparameterset = metapost.getparameterset

local isstart       = false

local p_whitespace  = lpeg.patterns.whitespace^1
local p_braces      = lpeg.patterns.argument
local p_parentheses = lpeg.patterns.nestedparents -- todo: remove outer

local f_wrapup      = formatters["  %s\n"]
local f_call_s      = formatters["  mfun_lsystem_%s(iteration)\n"]
local f_call_n      = formatters["  mfun_lsystem_%s(iteration-1)\n"]
local f_options     = formatters["  mfun_lsystem_options(%s)\n"]
local f_expression  = formatters["  mfun_lsystem_expression(%s)\n"]
----- f_expression  = formatters["  hide(%s)\n"]
local f_expression  = formatters["  hide%s\n"]

----- trace  = false
local report = logs.reporter("metapost")

-- trackers.register("metapost.lsystems", function(v) trace = v end) -- file has to be loaded

local function whatever(s)
    return isstart and f_call_s(s) or f_call_n(s)
end

local function options(s)
    return f_options(s)
end

local function expression(s)
    return f_expression(s)
end

local parser = Cs( (
      R("AZ","az")    / whatever
      + P("+")        / "  mfun_lsystem_plus\n"
      + P("-")        / "  mfun_lsystem_minus\n"
      + P("?")        / "  mfun_lsystem_move\n"
      + P("!")        / "  mfun_lsystem_draw\n"
      + P("[")        / "  mfun_lsystem_push\n"
      + P("]")        / "  mfun_lsystem_pop\n"
      + P("*")        / "  mfun_lsystem_close\n"
      + p_braces      / options
      + p_parentheses / expression
      + p_whitespace  / ""
) ^1 )

local definition = utilities.templates.replacer
[[
def mfun_lsystem_%name%(expr iteration) =
 if iteration == 0 :
%quit%
 else :
%step% fi
enddef ;
]]

function mp.lsystem_prepare()
    local parameters = getparameterset("lsystem")
    local syntax     = parameters.syntax
    local lsystem    = parameters.name or "unknown"
    local result     = { }
    local step       = nil
    local quit       = ""
    local isempty    = { }
    local trace      = parameters.trace
    parameters.isimage = false
    for name, pattern in next, syntax do
        if find(pattern,"{") then
            parameters.isimage = true
        end
        if pattern == "" or pattern == "!" or pattern == "?" then
            isempty[name] = pattern
            syntax [name] = ""
        end
    end
    for name, pattern in next, syntax do
        isstart = name == "start"
        step = lpegmatch(parser,pattern)
        if not step then
            step = "  mfun_lsystem_draw"
        end
        if isempty[name] then
            if isempty[name] == "!" then
                quit = "mfun_lsystem_draw"
            elseif isempty[name] == "?" then
                quit = "mfun_lsystem_move"
            else
                quit = ""
            end
        elseif next(isempty) then
            quit = ""
        else
            quit = find(name,"^[a-z]") and "  mfun_lsystem_move" or "  mfun_lsystem_draw"
        end
        step = definition {
            name = name,
            quit = quit,
            step = step,
        }
        result[#result+1] = step
    end
    result = concat(result,"\n")
    if trace then
        report("start l system %a\n\n%s",lsystem,result)
        report("stop l system %a",lsystem)
    end
--     print(result)
    return result
end