...and where can I find the documentation to HeNEXT ?

HeNEXT is a macro (surprise!), defined in hv.h as

#define HeNEXT(he)        (he)->hent_next
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where hent_next is a field in the following struct (also from hv.h):

typedef struct he HE;

/* ... */

/* entry in hash value chain */
struct he {
    HE          *hent_next;     /* next entry in chain */
    HEK         *hent_hek;      /* hash key */
    SV          *hent_val;      /* scalar value that was hashed */
};
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So basically HeNEXT scoots along a linked list of hash entries (those belonging to a given bucket, to be precise). As you can see, the data structure only provides for travel in one direction. Therefore, I don't see an easy way to implement a prev method. If I had to have it, I think I'd just accept the memory hit and use the keys array-based implementation I described briefly in my OP. Then prev reduces to decrementing a cursor variable.

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