on-number">15.5</span> Using external C++ code</h2>
<p>The <code>--allow_undefined</code> flag can be passed to the call to stanc,
which will allow undefined functions in the Stan language to be parsed
without an error. We can then include a definition of the function in
a C++ header file.
This requires specifying two makefile variables:
- <code>STANCFLAGS=--allow_undedefined</code>
- <code>USER_HEADER=&lt;header_file.hpp&gt;</code>, where <code>&lt;header_file.hpp&gt;</code> is the name of a header file th