A novel calculator, ideal for interactive whiteboards and pen-based devices, is introduced and evaluated. The calculator provides a natural, dynamic method of entering conventional expressions by handwriting and provides continual feedback showing the calculation and results. The user interface adjusts and copes with partial expressions, morphing the expressions to correct position and syntax. Gestures are also used to edit and manipulate calculations. The user interface is declarative, in that all displays, even with partial user input, are of correct calculations. We have found the user interface to be surprisingly engaging, and people enjoy using it and playing with mathematics.

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