This is documentation for Mathematica 3, which was
based on an earlier version of the Wolfram Language.
 On On[ symbol :: tag ] switches on a message, so that it can be printed. On[ s ] switches on tracing for the symbol s. On[ , , ... ] switches on several messages. On[ ] switches on tracing for all symbols. When tracing is switched on, each evaluation of a symbol, on its own, or as a function, is printed, together with the result. Note that the tracing information is printed when a function returns. As a result, traces of recursive functions appear in the opposite order from their calls. On[ s ] is equivalent to On[ s ::trace]. On[ ] is equivalent to On[ s ::trace] for all symbols. See the Mathematica book: Section 1.3.11, Section 2.8.21. See also: Off, TracePrint. Further Examples This switches on tracing of all symbols. In[1]:= On::trace: On[] --> Null. In[2]:= Set::trace: a = 5 --> 5. Out[2]= In[3]:= a::trace: a --> 5. 2 2Power::trace: a --> 5 . 2Power::trace: 5 --> 25. 2Plus::trace: a - c --> 25 - c. 2b::trace: b[a - c] --> b[25 - c]. Out[3]= This switches tracing off. In[4]:=