Allow simplification of inStream even if we find that mass-flow is ze…

…ro/negative

Closes modelica#2242

chapters/derivationofstream.tex

@@ -171,8 +171,10 @@ \subsection{Connection of 2 stream connectors, one to one connections (N=2):}\do
 \subsection{Connection of 3 stream connectors where one mass flow rate is identical to zero (N=3 and $\dot{m}_3=0$):}\doublelabel{connection-of-3-stream-connectors-where-one-mass-flow-rate-is-identical-to-zero-n-3-and}
 This case occurs, when a one-port sensor (like a temperature sensor) is
 connected to two connected components. For the sensor, the min attribute
-of the mass flow rate has to be set to zero (no fluid exiting the
-component via this connector). The suggested implementation results in
+of the mass flow rate should be set to zero (no fluid exiting the
+component via this connector). 
+This simplification can also be used if a tool determines that a mass flow rate is zero or non-negative.
+The suggested implementation results in
 the following equations, and as indicated the last formula can be
 simplified further by using $\dot{m}_3=0$:
 \begin{eqnarray*}