Convert a MATLAB script into symbolic script to find the symbolic derivative

Question

I have the following function in MATLAB.

function [curve_final] = RDM_FP(theta_in,T,t0)

Nf = length(T);
fs = 1/(T(2)-T(1));

that = T-t0;

pos_interval = that>0;
neg_interval = that<=0;
that = that(pos_interval);
%fitting parameters
alpha = theta_in(1);%parameter alpha from extended dispersion Tofts model alpha= a*Ktrans
gamma = theta_in(2)/60;%parameter gamma from extended dispersion Tofts model gamma=kep
kappa = theta_in(3);%parameter k from extended dispersion Tofts model (dispersion coefficient)
mu = theta_in(4); %parameter mu from extended dispersion Tofts model (Mean Transit Time parameter)

curve1(pos_interval) = sqrt(kappa./(2.*pi.*(that))).*exp(-kappa.*(that-mu).^2./(2.*(that)));
curve1(neg_interval) = 0;


curve2(pos_interval) = exp(-gamma.*(that));
curve2(neg_interval) = 0;

curve1 = [curve1 zeros(1,1000 -Nf)];
curve2 = [curve2 zeros(1,1000 -Nf)];

ft1 = fft(curve1);
ft2 = fft(curve2);

ft_total = ft1.*ft2;

curve= real(ifft(ft_total));

curve_final = 1/fs*alpha.*curve(1:Nf);
end

I want to convert this function into a symbolic script. I tried the following thing.

clear all,clc
syms theta_in [1 4] matrix
syms T [1 10]
syms t0

Nf = length(T);
fs = 1/(T(2)-T(1));

that = T-t0;

pos_interval = isAlways(that>0);
neg_interval = isAlways(that<=0);
that = that(pos_interval)
%fitting parameters
alpha = theta_in(1);%parameter alpha from extended dispersion Tofts model alpha= a*Ktrans
gamma = theta_in(2)/60;%parameter gamma from extended dispersion Tofts model gamma=kep
kappa = theta_in(3);%parameter k from extended dispersion Tofts model (dispersion coefficient)
mu = theta_in(4); %parameter mu from extended dispersion Tofts model (Mean Transit Time parameter)

curve1(pos_interval) = sqrt(kappa./(2.*pi.*(that))).*exp(-kappa.*(that-mu).^2./(2.*(that)));
curve1(neg_interval) = 0;


curve2(pos_interval) = exp(-gamma.*(that));
curve2(neg_interval) = 0;

curve1 = [curve1 zeros(1,1000 -Nf)];
curve2 = [curve2 zeros(1,1000 -Nf)];

ft1 = fft(curve1);
ft2 = fft(curve2);

ft_total = ft1.*ft2;

curve= real(ifft(ft_total));

curve_final = 1/fs*alpha.*curve(1:Nf);

However, I get the following error

I believe the problem lies in the line pos_interval = isAlways(that>0) since that is a symbol array and the condition cannot be evaluated for symbolic expressions. Can someone please provide some help. Thank you