an or equal to <CODE
CLASS="FUNCTION"
>PQntuples</CODE
>, meaning this
      function can only grow the tuples array one tuple at a time.  But any
      field of any existing tuple can be modified in any order.  If a value at
      <TT
CLASS="PARAMETER"
>field_num</TT
> already exists, it will be overwritten.
      If <TT
CLASS="PARAMETER"
>len</TT
> is -1 or
      <TT
CLASS="PARAMETER"
>value</TT
> is <TT
CLASS="SYMBOL"
>NULL</TT
>, the field value
      will be set to an SQL null value.  The
      <TT
CLA