sub write_make {
    
    # create lists of rates, states and tmpvars so that
    # separate rules can be created in the makefile
    my @list_of_rates;
    my @list_of_outputs;
    my @list_of_unknown;
    my @list_of_nonstates;
    my @list_of_tmpvars;

    for my $lvar (sort (keys %expressions)) {
	if ($lvar =~ /^MTTdX\(/) {
	    @list_of_rates = (@list_of_rates, $lvar) ;
	} elsif ($lvar =~ /^MTTy\(/) {
	    @list_of_outputs = (@list_of_outputs, $lvar);
	} elsif ($lvar =~ /^MTTyz/) {
	    @list_of_unknown = (@list_of_unknown, $lvar);
	} elsif ($lvar =~ /^MTTz/) {
	    @list_of_nonstates = (@list_of_nonstates, $lvar);
	} elsif ($lvar =~ /^${sys}_/) {
	    @list_of_tmpvars = (@list_of_tmpvars, $lvar);
	} else {
	    die "MTT Error:\nese_txt2make, unclassified variable: $lvar\n";
	}
    }
    my @sorted_rates   = sort (@list_of_rates);
    my @sorted_outputs = sort (@list_of_outputs);
    my @sorted_unknown = sort (@list_of_unknown);
    my @sorted_tmpvars = sort (@list_of_tmpvars);
    my @sorted_rates     = sort (@list_of_rates);
    my @sorted_outputs   = sort (@list_of_outputs);
    my @sorted_unknown   = sort (@list_of_unknown);
    my @sorted_nonstates = sort (@list_of_nonstates);
    my @sorted_tmpvars   = sort (@list_of_tmpvars);


    # write the header
    open (ESE, ">$outfile") or
	die ("MTT Error:\nese_txt2make, cannot open $outfile\n");

    my $date = localtime;

	"# Created by MTT: $date\n\n";    

    # write the rules that external programs use
    print ESE
        "all: declare_tmpvars MTTdX MTTy\n\n" .
	"MTTdX: @sorted_rates\n\n" .
	"MTTy:  @sorted_outputs\n\n" .
	"MTTyz: @sorted_unknown\n\n";
	"MTTyz: @sorted_unknown\n\n" .
	"MTTz:  @sorted_nonstates\n\n";
    
    # set the default output format:
    # double tmpvar;
    # lvalue := expression;
    print ESE
	"# default output format\n" .
	"ifeq (\"\$(assignment)\",\"\")\n" .