interface rearrangement

README.md

@@ -45,6 +45,13 @@ with exact component checks for selected representable cases. Near underflow,
 extended precision necessarily decreases; overflow produces infinity. Performance
 of the allocating exponent-boundary path is not covered by the basic benchmarks.
 
+Named value constants include `Zero`, `NegativeZero`, `One`, `NegativeOne`, `NaN`,
+`PositiveInfinity`, `NegativeInfinity`, and `Epsilon`. `Epsilon` is the smallest
+positive representable value, `2^-1074` (the same as `double.Epsilon`), **not** a
+relative-error tolerance or machine epsilon. These constants have a positive-zero
+low component; `NegativeZero` preserves the high sign bit while comparing and
+hashing equal to `Zero`.
+
 ## Predefined mathematical constants
 
 All constants below are static `DoubleDouble` properties. Each stores the nearest
@@ -52,6 +59,10 @@ binary64 high component followed by the nearest binary64 residual, rather than
 calculating a ratio, root, or logarithm on access. Names use PascalCase, including
 `Pi`, `E`, and `Ln2`.
 
+`DoubleDouble` implements `IFloatingPointConstants<DoubleDouble>` for generic
+access to `E`, `Pi`, and `Tau`; this does not imply support for the full
+`IFloatingPointIeee754<DoubleDouble>` interface.
+
 | Group | Properties and values |
 |---|---|
 | Circle and common angles | `Pi` (π), `Tau` (2π), `PiOver2`, `PiOver3`, `PiOver4`, `PiOver6` |
@@ -280,9 +291,9 @@ bool success = DoubleDouble.TryParse("1.25e-2".AsSpan(), CultureInfo.InvariantCu
 ## Deferred scope
 
 Natural `DDMath.Log`, `Exp`, and all three `Pow` overloads are implemented.
-Logarithms in other bases, generic-math interfaces beyond `ISignedNumber`,
-additional text formats/general round-trip formatting, and non-arithmetic performance
-benchmarks remain deferred.
+Logarithms in other bases, generic-math interfaces beyond `ISignedNumber` and
+`IFloatingPointConstants`, additional text formats/general round-trip formatting,
+and non-arithmetic performance benchmarks remain deferred.
 Replacing allocating arithmetic boundary fallbacks is also deferred; the current
 `BigInteger` paths remain in place. That optimization does not require removing
 `BigInteger` from conversions, parsing, formatting, or independent test oracles.