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5.1.6 Trace
If the Trace option is on, then the system echoes every command at every level. This is useful for debugging programs. The following (toy) user-defined function returns the same error message for N = 3 and N = 6. Turning the Trace option on makes the sources of the trouble clear.

Example 
  Define T(N)
    M := 1/(N-3);
    If N = 6 Then N := 3 EndIf;
    M := 1/(N-3);
    Return M;
  EndDefine;
  T(3);
ERROR: Division by zero
CONTEXT: 1 / (N - 3)
-------------------------------
  T(6);
ERROR: Division by zero
CONTEXT: 1 / (N - 3)
-------------------------------
  Set Trace;
  T(3);
T(3)
M := 1 / (N - 3); If N = 6 Then N := 3 EndIf; M := 1 / (N - 3); Return(M);
M := 1 / (N - 3)
ERROR: Division by zero
CONTEXT: 1 / (N - 3)
-------------------------------
  T(6);
T(6)
M := 1 / (N - 3); If N = 6 Then N := 3 EndIf; M := 1 / (N - 3); Return(M);
M := 1 / (N - 3)
If N = 6 Then N := 3 EndIf
N := 3
M := 1 / (N - 3)
ERROR: Division by zero
CONTEXT: 1 / (N - 3)
-------------------------------