Please read the following table in Markdown format and then answer the question according to the table. Table cells in one row are seperated by '|', and different rows are seperated by '
'.

Table:
| Series No. | Episode No. | Title | Directed by | Written by | UK Ratings (BBC2 Rank) | Original air date |
| --- | --- | --- | --- | --- | --- | --- |
| 4.0 | 1.0 | "Whitstable to the Isle of Wight " | David Symonds | n/a | 3.68m (2) | 14July2009 |
| 4.0 | 2.0 | " France : Cap Gris-Nez to Mont-Saint-Michel " | David Symonds | n/a | 3.34m (3) | 21July2009 |
| 4.0 | 3.0 | " Land's End to Porthcawl " | Marc Edwards | n/a | 3.45m (2) | 28July2009 |
| 4.0 | 4.0 | " Cork to Dublin " | John Trefor | n/a | 3.56m (2) | 4August2009 |
| 4.0 | 5.0 | " Anglesey to Blackpool " | Paul Barnett | n/a | 3.41m (1) | 11August2009 |
| 4.0 | 6.0 | " Inner Hebrides to the Faroe Islands" | Lizzie Mottram | n/a | 3.66m (1) | 18August2009 |
| 4.0 | 7.0 | " Norway : Lillesand to Svalbard " | Amanda Reilly | n/a | 3.66m (2) | 25August2009 |
Question:
Name the written by for john trefor
Answer:
n/a