I was trying to reproduce these tutorials in Python using scipy.signal:

http://ctms.engin.umich.edu/CTMS/index.php?example=CruiseControl&section=SystemAnalysis

but I found that I cannot multiply a system by a number to obtain a system whose transfer function is the original one multiplied by that number, neither can I multiply two systems to operate them as diagram blocks. It would be just a convenience enhancement but probably others will find it useful, specially if migrating from MATLAB.