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diff --git a/include/ruby/internal/method.h b/include/ruby/internal/method.h
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+#ifndef RBIMPL_METHOD_H                              /*-*-C++-*-vi:se ft=cpp:*/
+#define RBIMPL_METHOD_H
+/**
+ * @file
+ * @author     Ruby developers <ruby-core@ruby-lang.org>
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ * @warning    Symbols   prefixed  with   either  `RBIMPL`   or  `rbimpl`   are
+ *             implementation details.   Don't take  them as canon.  They could
+ *             rapidly appear then vanish.  The name (path) of this header file
+ *             is also an  implementation detail.  Do not expect  it to persist
+ *             at the place it is now.  Developers are free to move it anywhere
+ *             anytime at will.
+ * @note       To  ruby-core:  remember  that   this  header  can  be  possibly
+ *             recursively included  from extension  libraries written  in C++.
+ *             Do not  expect for  instance `__VA_ARGS__` is  always available.
+ *             We assume C99  for ruby itself but we don't  assume languages of
+ *             extension libraries.  They could be written in C++98.
+ * @brief      Creation and modification of Ruby methods.
+ */
+#include "ruby/internal/attr/nonnull.h"
+#include "ruby/internal/dllexport.h"
+#include "ruby/internal/value.h"
+#include "ruby/backward/2/stdarg.h"
+
+/**
+ * @defgroup  defmethod  Defining methods
+ *
+ * There are some APIs to define a method from C.
+ * These API takes a C function as a method body.
+ *
+ * ### Method body functions
+ *
+ * Method body functions must return a VALUE and
+ * can be one of the following form:
+ *
+ * #### Fixed number of parameters
+ *
+ * This form is a normal C function, excepting it takes
+ * a receiver object as the first argument.
+ *
+ * ```CXX
+ * static VALUE my_method(VALUE self, VALUE x, VALUE y);
+ * ```
+ *
+ * #### argc and argv style
+ *
+ * This form takes three parameters: argc, argv and self.
+ * self is the receiver. argc is the number of arguments.
+ * argv is a pointer to an array of the arguments.
+ *
+ * ```CXX
+ * static VALUE my_method(int argc, VALUE *argv, VALUE self);
+ * ```
+ *
+ * #### Ruby array style
+ *
+ * This form takes two parameters: self and args.
+ * self is the receiver. args is an Array object which
+ * contains the arguments.
+ *
+ * ```CXX
+ * static VALUE my_method(VALUE self, VALUE args);
+ * ```
+ *
+ * ### Number of parameters
+ *
+ * Method defining APIs takes the number of parameters which the
+ * method will takes. This number is called argc.
+ * argc can be:
+ *
+ *   - Zero or positive number.
+ *     This means the method body function takes a fixed number of parameters.
+ *
+ *   - `-1`.
+ *     This means the method body function is "argc and argv" style.
+ *
+ *   - `-2`.
+ *     This means the method body function is "self and args" style.
+ *
+ * @{
+ */
+
+RBIMPL_SYMBOL_EXPORT_BEGIN()
+
+RBIMPL_ATTR_NONNULL(())
+/**
+ * Defines a method.
+ *
+ * @param[out]  klass  A module or a class.
+ * @param[in]   mid    Name of the function.
+ * @param[in]   func   The method body.
+ * @param[in]   arity  The number of parameters.  See @ref defmethod.
+ * @note        There are in fact 18 different prototypes for func.
+ * @see         ::ruby::backward::cxxanyargs::define_method::rb_define_method
+ */
+void rb_define_method(VALUE klass, const char *mid, VALUE (*func)(ANYARGS), int arity);
+
+RBIMPL_ATTR_NONNULL(())
+/**
+ * Defines a module function for a module.
+ *
+ * @param[out]  klass  A module or a class.
+ * @param[in]   mid    Name of the function.
+ * @param[in]   func   The method body.
+ * @param[in]   arity  The number of parameters.  See @ref defmethod.
+ * @note        There are in fact 18 different prototypes for func.
+ * @see         ::ruby::backward::cxxanyargs::define_method::rb_define_module_function
+ */
+void rb_define_module_function(VALUE klass, const char *mid, VALUE (*func)(ANYARGS), int arity);
+
+RBIMPL_ATTR_NONNULL(())
+/**
+ * Defines a global function.
+ *
+ * @param[in]  mid    Name of the function.
+ * @param[in]  func   The method body.
+ * @param[in]  arity  The number of parameters.  See @ref defmethod.
+ * @note       There are in fact 18 different prototypes for func.
+ * @see        ::ruby::backward::cxxanyargs::define_method::rb_define_global_function
+ */
+void rb_define_global_function(const char *mid, VALUE (*func)(ANYARGS), int arity);
+
+RBIMPL_ATTR_NONNULL(())
+/**
+ * Defines an undef of a method.  -- What?
+ *
+ * In ruby, there are two separate concepts called "undef" and "remove_method".
+ * The thing you imagine when you  "un-define" a method is remove_method.  This
+ * one on the  other hand is masking of a  previous method definition.  Suppose
+ * for instance:
+ *
+ * ```ruby
+ * class Foo
+ *   def foo
+ *   end
+ * end
+ *
+ * class Bar < Foo
+ *   def bar
+ *     foo
+ *   end
+ * end
+ *
+ * class Baz < Foo
+ *   undef foo            # <--- (*1)
+ * end
+ * ```
+ *
+ * This `undef foo` at `(*1)` must not eliminate `Foo#foo`, because that method
+ * is also used from `Bar#bar`.  So  instead of physically executing the target
+ * method, `undef` inserts  a special filtering entry to the  class (`Baz` this
+ * case).  That entry,  when called, acts as  if there were no  methods at all.
+ * But the original can still be accessible, via ways like `Bar#bar` above.
+ *
+ * @param[out]  klass            The class to insert an undef.
+ * @param[in]   name             Name of the undef.
+ * @exception   rb_eTypeError    `klass` is a non-module.
+ * @exception   rb_eFrozenError  `klass` is frozen.
+ * @see         rb_remove_method
+ */
+void rb_undef_method(VALUE klass, const char *name);
+
+RBIMPL_ATTR_NONNULL(())
+/**
+ * Defines an alias of a method.
+ *
+ * @param[in,out]  klass            The class which the original method belongs
+ *                                  to; this is also  where the new method will
+ *                                  belong to.
+ * @param[in]      dst              A new name for the method.
+ * @param[in]      src              The original name of the method.
+ * @exception      rb_eTypeError    `klass` is a non-module.
+ * @exception      rb_eFrozenError  `klass` is frozen.
+ * @exception      rb_eNameError    There is  no such method named  as `src` in
+ *                                  `klass`.
+ *
+ * @internal
+ *
+ * Above  description  is   in  fact  a  bit  inaccurate   because  it  ignores
+ * Refinements.
+ */
+void rb_define_alias(VALUE klass, const char *dst, const char *src);
+
+RBIMPL_ATTR_NONNULL(())
+/**
+ * Defines public accessor method(s) for an attribute.
+ *
+ * @param[out]  klass            The class which the attribute will belong to.
+ * @param[in]   name             Name of the attribute.
+ * @param[in]   read             Whether to define a getter method.
+ * @param[in]   write            Whether to define a setter method.
+ * @exception   rb_eTypeError    `klass` is a non-module.
+ * @exception   rb_eFrozenError  `klass` is frozen.
+ * @exception   rb_eNameError    `name` invalid as an attr e.g. an operator.
+ */
+void rb_define_attr(VALUE klass, const char *name, int read, int write);
+
+/** @} */
+
+RBIMPL_SYMBOL_EXPORT_END()
+
+#endif /* RBIMPL_METHOD_H */