Build a small four-operation calculator with a visible expression and result.
Offer input-order and multiplication/division-first evaluation rules.
Use exact decimal or fraction arithmetic so 0.1 + 0.2 displays 0.3.
Show 2 + 3 × 4 as 20 in input order and 14 with precedence.
Show 123456789 × 987654321 exactly as 121932631112635269.
For repeating fractions, display at most 16 decimal places with half-up rounding.
Mark rounded results. Keep intermediate arithmetic exact.
Changing the expression or rule must clear the previous result.
Handle division by zero with a useful message.
Provide Clear, backspace, keyboard calculation and visible calculation steps.
Verify Clear after an error allows 7 + 2 to return 9.
Include run instructions and the actual checks performed.