function [a,g,b,c] = mapABCD(ABCD,form)%[a,g,b,c] = mapABCD(ABCD,form='CRFB')%Compute the coefficients for the specified structure.%See realizeNTF.m for a list of supported structures.%stuffABCD is the inverse function.% Code common to all structures.if nargin<2    if nargin<1		fprintf(1,'%s error: Insufficient arguments (%.0f).\n',mfilename,nargin)		return    else		form = 'CRFB';    endendorder=size(ABCD,1)-1;odd = rem(order,2); even = ~odd;diagonal = 1:(order+2):order*(order+1);subdiag = diagonal(1:order)+1;supdiag = diagonal(2+odd:2:order)-1;%Do the mapping.switch formcase {'CRFB', 'CIFB', 'CRFBD'}    c = ABCD(subdiag);    g = -ABCD(supdiag);    if strcmp(form,'CRFB') 		dly = (2+odd):2:order;		%row numbers of delaying integrators.		ABCD(dly,:) = ABCD(dly,:)-diag(c(dly-1))*ABCD(dly-1,:);    elseif strcmp(form,'CRFBD') 		dly = (1+odd):2:order;		%row numbers of delaying integrators.		ABCD(dly,:) = ABCD(dly,:)+diag(g)*ABCD(dly+1,:);		if order>2		    coupl = 2+even:2:order;		    ABCD(coupl,:) = ABCD(coupl,:) - diag(c(coupl-1))*ABCD(coupl-1,:);		end    end    a = -ABCD(1:order,order+2)';    b = ABCD(:,order+1)';case 'CRFF'    c = [-ABCD(1,order+2) ABCD(subdiag(1:end-1))];    g = -ABCD(supdiag);    if even        multg = 1:2:order;	%Rows to have g*(following row) added.        ABCD(multg,:) = ABCD(multg,:)+diag(g)*ABCD(multg+1,:);    end    multc=3:2:order;	%Rows to have c*(preceding row) subtracted.    ABCD(multc,:)=ABCD(multc,:)-diag(c(multc))*ABCD(multc-1,:);    a(2:2:order)=ABCD(order+1,2:2:order);    for i=2:2:order                   %Recover a coeff.        ABCD(order+1,:)=ABCD(order+1,:)-a(i)*ABCD(i,:);    end    a(1:2:order)=ABCD(order+1,1:2:order);    b = ABCD(:,order+1)';case 'CRFFD'    % CRFFD has an extra order. Correct the common variables    order = order-1;    odd = rem(order,2); even = ~odd;    diagonal = diagonal(1:order);    subdiag = diagonal(1:order-1)+1;    supdiag = diagonal(2+odd:2:order)-1;    g = -ABCD(supdiag);    c = [-ABCD(1,order+3) ABCD(subdiag)];    a = zeros(1,order);    for i=1:2:order                   %Recover the a coefficients		a(i) = ABCD(order+1,i);		ABCD(order+1,:) = ABCD(order+1,:) - a(i)*ABCD(i,:);    end    a(2:2:order) = ABCD(order+1,2:2:order);    b = ABCD(1:order+1,order+2)';    for i=2:2:order                   %Recover the b coefficients		b(i) = b(i) - c(i) *b(i-1);		if odd		    b(i) = b(i) + g(i/2) *b(i+1);		end    end    yscale =  ABCD(order+2,order+1);    a = a*yscale;    b(end) = b(end)*yscale;case 'CIFF'    a = ABCD(order+1,1:order);    c = [-ABCD(1,order+2) ABCD(subdiag(1:end-1))];    g = -ABCD(supdiag);    b = ABCD(:,order+1)';otherwise    fprintf(1,'%s error: Form %s is not yet supported.\n',mfilename,form)end