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:
| Co-king | Relationship to Monarch | Crowned | Co-kingship ceased | Reason | Monarch |
| --- | --- | --- | --- | --- | --- |
| Robert (II) | son | 30 December 987 | 24 October 996 | became sole king | Hugh Capet |
| Hugh Magnus | son | 19 June 1017 | 17 September 1025 | died | Robert II of France |
| Henry (I) | son | 14 May 1027 | 20 July 1031 | became sole king | Robert II of France |
| Philip (I) | son | 23 May 1059 | 4 August 1060 | became sole king | Henry I of France |
| Philip | son | 14 April 1129 | 13 October 1131 | died | Louis VI of France |
| Louis (VII) | son | 25 October 1131 | 1 August 1137 | became sole king | Louis VI of France |
Question:
Name the reason for 20 july 1031
Answer:
became sole king