Assertions are used to find errors in algorithms. They test whether intermediate results conform to required conditions and issue an error if not.
SetAssertionLevel(lev) F
assigns the global assertion level to lev. By default it is zero.
AssertionLevel() F
returns the current assertion level.
Assert(lev,cond) F
if the global assertion level is at least lev, condition cond is tested
and if it does not return true an error is raised.
Thus Assert(lev, code) is equivalent to the code
if AssertionLevel() >= lev and not <code> then
Error("Assertion failure");
fi;
Assert(lev, cond, message ) F
In this version of the Assert statement, if the global assertion
level is at least lev, condition cond is tested and if it does not
return true then message is evaluated and printed.
Assertions are used at various places in the library. Thus turning assertions on can slow code execution significantly.
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GAP 4 manual