isstable (sys, tol)     #

feedback (sys)          # equivalent to feedback (sys, eye (size (sys)))
feedback (sys, "+")     # equivalent to feedback (sys, eye (size (sys)), +1)

H = sys(w)              # equivalent to H = freqresp (sys, w) where w is a frequency vector

place (sys, p, alpha)   # If parameter alpha is specified, poles with real parts
place (a, b, p, alpha)  # (continuous time) or moduli (discrete time) below alpha
                        # are left untouched.

sminreal (sys, tol)     # Optional tolerance for controllability and observability.
                        # Entries of the state-space matrices whose moduli are less or
                        # equal to TOL are assumed to be zero. Default value is 0.

frd (sys)               # calculate interesting frequency range by the zeros and poles of SYS.