When a command fails in URBI, it will send an error message, prefixed by three exclamation marks:

impossible:1/0;
[167322:impossible] !!! Division by zero
[167322:impossible] !!! EXPR evaluation failed

Note that the tag of the command is used in the error message, which is extremely convenient to know what has failed in a complex program.

Error messages are different from system messages prefixed by three stars, and which usually display information normally outputted or requested by the command. A typical example is echo with a +begin and +end flag:

mytag+begin+end:echo "hello there!";
[146711:mytag] *** begin
[146711:mytag] *** hello there!
[146711:mytag] *** end