Add unary -op for UDLs to work nicely.

graf2d/gpad/v7/inc/ROOT/TPadCoord.hxx

@@ -33,6 +33,8 @@ protected:
       CoordSysBase(double val): fVal(val) {}
       DERIVED &ToDerived() { return static_cast<DERIVED &>(*this); }
 
+      DERIVED operator-() { return DERIVED(-fVal); }
+
       friend DERIVED operator+(DERIVED lhs, DERIVED rhs) { return DERIVED{lhs.fVal + rhs.fVal}; }
       friend DERIVED operator-(DERIVED lhs, DERIVED rhs) { return DERIVED{lhs.fVal - rhs.fVal}; }
       friend double operator/(DERIVED lhs, DERIVED rhs) { return lhs.fVal / rhs.fVal; }
@@ -124,6 +126,11 @@ public:
       return TPadCoord{lhs.fNormal - rhs.fNormal, lhs.fPixel - rhs.fPixel, lhs.fUser - rhs.fUser};
    }
 
+   /// Unary -.
+   TPadCoord operator-() {
+      return TPadCoord(-fNormal, -fPixel, -fUser);
+   }
+
    /// Add a `TPadCoord`.
    TPadCoord &operator+=(const TPadCoord &rhs)
    {