Prerequisites | Step 1 — Pick a source file to mutate | Step 2 — Define a test plan | Step 3 — Run the tests | Step 4 — Read the output and improve your tests | The feedback loop | Next steps

Operator mutators | operator() — custom pair | arithmetic_operators() — preset for arithmetic | comparison_operators() — preset for comparisons | logical_operators() — preset for logical operators | Boolean literal mutators | boolean_literal() — flip TRUE/FALSE | boolean_literals() — preset for boolean literals | Numeric mutators | numeric_increment() — add 1 to every numeric literal | numeric_decrement() — subtract 1 from every numeric literal | numeric_literals() — preset for numeric constants | String mutators | string_empty() — replace strings with "" | string_fill() — replace empty strings with "mutant" | string_literals() — preset for string literals | Condition mutators | negate_condition() — wrap condition in !(...) | remove_condition_negation() — strip leading ! | remove_negation() — remove ! anywhere | condition_mutations() — preset for condition logic | Function call mutator | call_name() — swap one function name for another | NA and NULL mutators | na_literal() — swap NA/NULL values | na_literals() — preset for NA and NULL | Return value mutators | replace_return_value() — replace explicit return values | Index and subscript mutators | index_increment() — shift subscript indices up by one | index_decrement() — shift subscript indices down by one | index_mutations() — preset for subscript indices | Recommended starting set

Why run mutation tests in CI | GitHub Actions: minimal workflow | Mutation score badge | Choosing which files to mutate in CI | Failing the build on a low score | Performance tips

The output table | What the score tells you | Summary: three patterns, three fixes

Code coverage is not test quality | What a mutant is | Kill vs survive | The mutation score | The LLM-generated tests problem | When mutation testing pays off most | When it is less useful | How it relates to covr | Next steps

How BDD works | Capturing a wish with a User Story | Refining the User Story into examples | Implementing executable specifications | BDD with base R | BDD with | Why should you choose {cucumber}? | Learning BDD

Keywords | Feature | Descriptions | Example | Steps | Given | When | Then | And, But | * | Background | Tips for using Background | Scenario Outline | Examples | Step Arguments | Doc Strings | Data Tables | Table Cell Escaping

Step execution order | Translation to testthat code | How step definitions are loaded

1. Remove test-cucumber.R file. | 2. Move steps definitions to setup-*.R files.