From language/operators

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

Numeric context operator.

Coerces the argument to Numeric by calling the Numeric method on it.

From language/operators

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

Addition operator: Coerces both arguments to Numeric and adds them. From version 6.d it works also on Duration, DateTime and Real types.