opaque Abstract Data Types, especially if implemented in C) matching this name are exe- cuted. For example, if you define the type: NEWTYPE opaquel OPERATORS 286 Validation of Communications Systems with SDL print1: opaquel -> Boolean; OPERATOR print1; FPAR p1 opaque1; RETURNS res Boolean; START; WRITELN ('*** Hello world!'); RETURN True; ENDOPERATOR ; ENDNEWTYPE; If you declare a variable X of type opaquel in the SDL model, when you enter the command print X, you get no result because the Simulator does not know what to print (because the NEWTYPE opaquel is neither a struct, nor a literals list, nor an array etc.). If you enter define print_hook 'pr*', then typing print X activates the operator printl and Hello world! is displayed. define print_stop_condition_errors 'false': do not use. define range_check 'false': if true, the range overflow is checked when the Simulator evaluates a command containing ranges. define real_prec '6': number of digits after the decimal point for real numbers. define reasonable feed 'true': see Chapter 4. define run_forever 'false': do not use. define scc_sink_limit '2': maximum number of livelock scenario files generated during an exhaustive simulation, here 2. define show_optionals 'false': prevents the Simulator from displaying the OPTIONAL fields (generally from ASN.1) that are not present in the value of the sequence. define significance_level '0.05': used for performance simulation. define states_dump ' ': if a name is specified between the single quotes, the states of the states graph are written into the file name. For training only. define states_limit '0': maximum number of explored (unique) global states. When the limit is reached, the exhaustive simulation stops. 0 means no limit. In supertrace mode, it specifies the number of bits in the hash table, which must be 50 or 100 times the number of global system states to minimize the collision risk. define stop_cut 'true': if true, the states after a state where a stop condition is satisfied are not explored (cut is equivalen