Tips for golfing in D2

D is JavaScript.

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Obviously not. But, when dealing with float, double and real, nan is a value which must be handled, and, as specified by IEEE 754, NaN does not compare equal with itself.

writeln(double.nan == double.nan); // false

Meaning,

writeln(value!=value); // only true for nan

is way shorter than

import std.math;
writeln(isNaN(value));

Obviously, always use math.isNaN for real code. ;)