This is a unique product for a small touch screen devices that really unleashes the Power of Touch Interface.
The Gesture Language is built on micro-interactions and is nailed down to the following: Swipe left and right to navigate through Function/Conversion/Operation keypads (swipe down for Search/History), Tap to Add/Delete, Force Touch or Long-Press to Edit or Open.
The Design Language is based on Tokens. All tokens are of the same structure: the left part shows a type of the element, the right part has a result or an entered value and its name on top. In addition to tokens, there are pure numbers and math operators.
A user can set an expression as a variable to reuse in other expressions and custom-name the components of the calculation to make them self-explanatory and consistent at an unlimited number of nesting levels. Coherence of the system makes it amazingly powerful while ingeniously simple.