Use when defining or modifying MPS generators — author a generator module, add or edit root/reduction/weaving/pattern mapping rules, attach template macros ($COPY_SRC, $LOOP, $IF, $PROPERTY, $REF, $SWITCH, $MAP_SRC, $WEAVE, $INSERT, $LABEL, $TRACE, $VAR), wire mapping labels, build template switches, write pre/post mapping scripts, navigate `genContext`, or debug "rule didn't fire", missing references, empty output, infinite reduction loops, and generated-Java compile failures.