To prevent the numerator and denominator from becoming pathologically large, the denominator is limited to 64 bit storage

Well, that doesn't seem to be the case with (latest) rakudo-star-2017.07: 
> my $x=9.999999999999999999999999999999999999999999999.Rat; $x.nude
(9999999999999999999999999999999999999999999999 1000000000000000000000
+000000000000000000000000)
> $x.WHAT
(Rat)

[download]
but the base 10 denominator of 1000000000000000000000000000000000000000000000 is a 150-bit value.

The documentation also concludes with "Rat objects are immutable" - yet the following operation converts $x from a Rat to a FatRat: 
> $x *= 0.3.FatRat; $x.nude
(29999999999999999999999999999999999999999999997 100000000000000000000
+00000000000000000000000000)
> $x.WHAT
(FatRat)

[download]
Am I misunderstanding that documentation ?
Is there a way of interpreting the documentation such that it does match the above behaviours ?

Cheers,
Rob

In reply to Re^7: Small Perl 6 discoveries II, Rats by syphilis
in thread [Perl6] Small discoveries I, __DATA__ by holli

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