MATLAB: How do i ensure that the correct function is called when the same function is in multiple toolboxes

Control System Toolboxmultiple functions; ss

the function ss appears to me included in a number of toolboxes
>> which -all ss
/Applications/MATLAB_R2016b.app/toolbox/ident/ident/@idParametric/ss.m              % idParametric method
/Applications/MATLAB_R2016b.app/toolbox/shared/controllib/engine/@StaticModel/ss.m  % StaticModel method
/Applications/MATLAB_R2016b.app/toolbox/signal/signal/@dfilt/ss.m                   % dfilt method
when running the following i get this error msg; "Undefined function 'ss' for input arguments of type 'double'"
>> tau = 5;
K = 2;
A = -1/tau;
B = K/tau;
C = 1;
D = 0;
>> 
>> sys = ss(A,B,C,D)
Undefined function 'ss' for input arguments
of type 'double'.

Best Answer

>> which -all ss
/Applications/MATLAB_R2016b.app/toolbox/control/ctrlmodels/@ss/ss.m                 % ss constructor
/Applications/MATLAB_R2016b.app/toolbox/signal/signal/@dfilt/ss.m                   % dfilt method
/Applications/MATLAB_R2016b.app/toolbox/control/ctrlmodels/@DynamicSystem/ss.m      % DynamicSystem method
/Applications/MATLAB_R2016b.app/toolbox/shared/controllib/engine/@StaticModel/ss.m  % StaticModel method
/Applications/MATLAB_R2016b.app/toolbox/mpc/mpc/@mpc/ss.m                           % mpc method
You are missing the first of those, the Control System Toolbox class constructor of ss . It appears to me that you might not have that toolbox installed.
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