#ifndef _LISP_H                                                                                                                                                                                                     
#define _LISP_H                                                                                                                                                                                                     
                                                                                                                                                                                                                    
#define MAX_NODES 100           /* essentially, the number of fun args */                                                                                                                                           
                                                                                                                                                                                                                    
typedef union                   /* Contains all types of data. */                                                                                                                                                   
{                                                                                                                                                                                                                   
    int i;                                                                                                                                                                                                          
    double d;                                                                                                                                                                                                       
    char c,                                                                                                                                                                                                         
        * s;                                                                                                                                                                                                        
} u_type;                                                                                                                                                                                                           
                                                                                                                                                                                                                    
typedef union                   /* an S-expression structure. */                                                                                                                                                    
{                                                                                                                                                                                                                   
    u_type v;                                                                                                                                                                                                       
    s_exp* n[MAX_NODES];                                                                                                                                                                                            
} s_exp;                                                                                                                                                                                                            
#endif