anchor.diff:              __spliced =
                    (lib.optionalAttrs (pkgsBuildBuild ? ${name}) { buildBuild = valueBuildBuild; })
                 // (lib.optionalAttrs (pkgsBuildTarget ? ${name}) { buildTarget = valueBuildTarget; })
  -              // { hostHost = valueHostHost; }
  +              // (lib.optionalAttrs (pkgsHostHost ? ${name}) { hostHost = valueHostHost; })
                 // (lib.optionalAttrs (pkgsTargetTarget ? ${name}) { targetTarget = valueTargetTarget;
             });
           };
  
positive.code:         valueBuildBuild = pkgsBuildBuild.${name} or {};
          valueBuildHost = pkgsBuildHost.${name} or {};
          valueBuildTarget = pkgsBuildTarget.${name} or {};
          valueHostHost = throw "`valueHostHost` unimplemented: pass manually rather than relying on splice.";
          valueHostTarget = pkgsHostTarget.${name} or {};
          valueTargetTarget = pkgsTargetTarget.${name} or {};
          augmentedValue = defaultValue
  
negative.code:           pkgsBuildBuild = valueBuildBuild;
            pkgsBuildHost = valueBuildHost;
            pkgsBuildTarget = valueBuildTarget;
            pkgsHostHost = valueHostHost;
            pkgsHostTarget = valueHostTarget;
            pkgsTargetTarget = valueTargetTarget;
          # Don't be fancy about non-derivations. But we could have used used
  