User Story #1: My calculator should contain a clickable element containing an = (equal sign) with a corresponding id="equals".
User Story #2: My calculator should contain 10 clickable elements containing one number each from 0-9, with the following corresponding IDs: id="zero", id="one", id="two", id="three", id="four", id="five", id="six", id="seven", id="eight", and id="nine".
User Story #3: My calculator should contain 4 clickable elements each containing one of the 4 primary mathematical operators with the following corresponding IDs: id="add", id="subtract", id="multiply", id="divide".
User Story #4: My calculator should contain a clickable element containing a . (decimal point) symbol with a corresponding id="decimal".
User Story #5: My calculator should contain a clickable element with an id="clear".
User Story #6: My calculator should contain an element to display values with a corresponding id="display".
User Story #7: At any time, pressing the clear button clears the input and output values, and returns the calculator to its initialized state; 0 should be shown in the element with the id of display.
User Story #8: As I input numbers, I should be able to see my input in the element with the id of display.
User Story #9: In any order, I should be able to add, subtract, multiply and divide a chain of numbers of any length, and when I hit =, the correct result should be shown in the element with the id of display.
User Story #10: When inputting numbers, my calculator should not allow a number to begin with multiple zeros.
User Story #11: When the decimal element is clicked, a . should append to the currently displayed value; two . in one number should not be accepted.
User Story #12: I should be able to perform any operation (+, -, *, /) on numbers containing decimal points.
User Story #13: If 2 or more operators are entered consecutively, the operation performed should be the last operator entered (excluding the negative (-) sign). For example, if 5 + _ 7 = is entered, the result should be 35 (i.e. 5 _ 7); if 5 * - 5 = is entered, the result should be -25 (i.e. 5 x (-5)).
User Story #14: Pressing an operator immediately following = should start a new calculation that operates on the result of the previous evaluation.
User Story #15: My calculator should have several decimal places of precision when it comes to rounding (note that there is no exact standard, but you should be able to handle calculations like 2 / 7 with reasonable precision to at least 4 decimal places).