Generate code for the following task step.

The task_step block below must specify: target file path, full symbol
signature, behaviour, concrete types (with import paths), forbidden
imports/constructs, imports already present in the target module, and
the exact output format. If any of these is missing or ambiguous, treat
that as a hard constraint to NOT introduce assumptions — emit only what
the task explicitly directs.

task_step:
{task_step}

Provide a complete, working implementation matching the format the task
requested. Do not wrap in markdown code fences. Do not introduce imports
that the task did not authorise.
