/*
 *  call-seq:
 *     term_enum.set_field(field) -> self
 *
 *  Set the field for the term_enum. The field value should be a symbol as
 *  usual. For example, to scan all title terms you'd do this;
 *
 *    term_enum.set_field(:title).each do |term, doc_freq|
 *      do_something()
 *    end
 */
static VALUE
frt_te_set_field(VALUE self, VALUE rfield)
{
    TermEnum *te = (TermEnum *)DATA_PTR(self);
    int field_num = 0;
    VALUE rfnum_map = rb_ivar_get(self, id_fld_num_map);
    VALUE rfnum = rb_hash_aref(rfnum_map, rfield);
    if (rfnum != Qnil) {
        field_num = FIX2INT(rfnum);
        rb_ivar_set(self, id_field_num, rfnum);
    } else {
        Check_Type(rfield, T_SYMBOL);
        rb_raise(rb_eArgError, "field %s doesn't exist in the index",
                 frt_field(rfield));
    }
    te->set_field(te, field_num);

    return self;
}