Closes #2253

Clarified instream text to say that assuming positive mass-flow.

chapters/derivationofstream.tex

@@ -128,7 +128,7 @@ \section{Rationale for the formulation of the inStream() operator}\doublelabel{r
 it is therefore the mixing enthalpy under the assumption of fluid
 flowing into said model.
 
-We establish this quantity using a dedicated operator $inStream(h_{outflow,i})=h_{mix}$ $(\dot{m}_{i} >= 0)$. This leads to
+We establish this quantity using a dedicated operator $inStream(h_{outflow,i})=h_{mix}$ assuming that $\dot{m}_{i} >= 0$. This leads to
 three different incarnations of (n in the general case). This is
 illustrated in the figure below. For the present example of three
 components in a connection set, this means the following.