Append every top-level section of another PomBuilder as a subsection of a
target section in this builder. Useful for composing prompts assembled
separately into a single document.
Throws Error("No section with title '<target>' found.") when target is a
string and no section with that title exists. For deeper lookups, resolve the
section yourself via findSection() and pass the
PomSection directly.
The target section to append into. Pass a title string for top-level
sections, or a PomSection instance (from findSection()) for nested
sections.
The builder whose sections should be appended.
this — returns the builder for fluent chaining.