Description:

• externalproc allows for binding the Sollya identifier identifier to an external code. After this binding, when Sollya encounters identifier applied to a list of actual parameters, it will evaluate these parameters and call the external code with these parameters. If the external code indicated success, it will receive the result produced by the external code, transform it to Sollya's internal representation and return it.
  
  In order to allow correct evaluation and typing of the data in parameter and in result to be passed to and received from the external code, externalproc has a third parameter argumenttype -> resulttype. Both argumenttype and resulttype are one of void, constant, function, range, integer, string, boolean, list of constant, list of function, list of range, list of integer, list of string, list of boolean.
  
  If upon a usage of a procedure bound to an external procedure the type of the actual parameters given or its number is not correct, Sollya produces a type error. An external function not applied to arguments represents itself and prints out with its argument and result types.
  
  The external function is supposed to return an integer indicating success. It returns its result depending on its Sollya result type as follows. Here, the external procedure is assumed to be implemented as a C function.
  
  If the Sollya result type is void, the C function has no pointer argument for the result. If the Sollya result type is constant, the first argument of the C function is of C type mpfr_t *, the result is returned by affecting the MPFR variable. If the Sollya result type is function, the first argument of the C function is of C type node**, the result is returned by the node * pointed with a new node *. If the Sollya result type is range, the first argument of the C function is of C type sollya_mpfi_t *, the result is returned by affecting the interval-arithmetic variable. If the Sollya result type is integer, the first argument of the C function is of C type int *, the result is returned by affecting the int variable. If the Sollya result type is string, the first argument of the C function is of C type char **, the result is returned by the char * pointed with a new char *. If the Sollya result type is boolean, the first argument of the C function is of C type int *, the result is returned by affecting the int variable with a boolean value. If the Sollya result type is list of type, the first argument of the C function is of C type chain **, the result is returned by the chain * pointed with a new chain *. This chain contains for Sollya type constant pointers mpfr_t * to new MPFR variables, for Sollya type function pointers node * to new nodes, for Sollya type range pointers sollya_mpfi_t * to new interval-arithmetic variables, for Sollya type integer pointers int * to new int variables for Sollya type string pointers char * to new char * variables and for Sollya type boolean pointers int * to new int variables representing boolean values.
  
  The external procedure affects its possible pointer argument if and only if it succeeds. This means, if the function returns an integer indicating failure, it does not leak any memory to the encompassing environment.
  
  The external procedure receives its arguments as follows: If the Sollya argument type is void, no argument array is given. Otherwise the C function receives a C void ** argument representing an array of size equal to the arity of the function where each entry (of C type void *) represents a value with a C type depending on the corresponding Sollya type. If the Sollya type is constant, the C type the void * is to be casted to is mpfr_t *. If the Sollya type is function, the C type the void * is to be casted to is node *. If the Sollya type is range, the C type the void * is to be casted to is sollya_mpfi_t *. If the Sollya type is integer, the C type the void * is to be casted to is int *. If the Sollya type is string, the C type the void * is to be casted to is char *. If the Sollya type is boolean, the C type the void * is to be casted to is int *. If the Sollya type is list of type, the C type the void * is to be casted to is chain *. Here depending on type, the values in the chain are to be casted to mpfr_t * for Sollya type constant, node * for Sollya type function, sollya_mpfi_t * for Sollya type range, int * for Sollya type integer, char * for Sollya type string and int * for Sollya type boolean.
  
  The external procedure is not supposed to alter the memory pointed by its array argument void **.
  
  In both directions (argument and result values), empty lists are represented by chain * NULL pointers.
  
  In contrast to internal procedures, externally bounded procedures can be considered to be objects inside Sollya that can be assigned to other variables, stored in list etc.