e current packet.
:param flds: is a list of tuples (`fld`, `cond`) or (`fld`, `cond`, `hint`)
             where `fld` if a field type, and `cond` a "condition" to
             determine if `fld` is the field type that should be used.

``cond`` is either:

- a callable `cond_pkt` that accepts one argument (the packet) and
  returns True if `fld` should be used, False otherwise.
- a tuple (`cond_pkt`, `cond_pkt_val`), where `cond_pkt` is the same
  as in the previous case and `cond_pkt_val` is a callable that
  accepts two arguments (the packet, and the value to be set) and
  returns True if `fld` should be used, False otherwise.

See scapy.layers.l2.ARP (type "help(ARP)" in Scapy) for an example of
use.
)