Function xmlXPathRegisterFunc 

#[unsafe(no_mangle)]
pub unsafe extern "C" fn xmlXPathRegisterFunc(
    _ctxt: *mut _xmlXPathContext,
    _name: *const xmlChar,
    _f: Option<unsafe extern "C" fn(*mut c_void, c_int)>,
) -> c_int

Register an XPath function.

UPSTREAM-PARITY

int xmlXPathRegisterFunc(xmlXPathContextPtr ctxt,
                         const xmlChar *name, xmlXPathFunction f);