From language/operators

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multi sub prefix:<->(Any --> Numeric:D)

Negative numeric context operator.

Coerces the argument to Numeric by calling the Numeric method on it, and then negates the result.

From language/operators

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multi sub infix:<->($a, $b --> Numeric:D)

Subtraction operator: Coerces both arguments to Numeric and subtracts the second from the first. From version 6.d it works also on Duration, DateTime and Real types.