- defined for lerp between two fields, either with a constant or a field of interpolation factors. * plain Field, DimensionedField, FieldField, GeometricFields - using a field to lerp between two constants is not currently supported
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4340.html
- easier to clean, avoid confusion with 'real' applications, etc.