Added answer to 'Gap 6'

WACCLangSpec.tex

@@ -446,7 +446,7 @@ or an expression enclosed by parenthesis.
 We discuss the meaning of each of these expressions in more detail below.
 
 The expressions of the WACC language have been chosen to be side-effect free.
-\hl{... ??? ...}
+\hl{ As such, the program state will never be modified as a direct consequence of evaluating an expression.}
 \fillgap{\hl{Define side-effect free expressions}}{1 mark}
 
 \paragraph{Integer Literals:}