CREATE TABLE table_58171 ("Heir" TEXT, "Status" TEXT, "Relationship to Monarch" TEXT, "Became heir" TEXT, "Reason" TEXT, "Ceased to be heir" TEXT, "Monarch" TEXT)

Who is the Monarch whose Heir is Robert Curthose when the Reason is that the father became king?
SELECT "Monarch" FROM table_58171 WHERE "Heir" = 'robert curthose' AND "Reason" = 'father became king'
SELECT "Monarch" FROM "table_58171" WHERE "Heir" = 'robert curthose' AND "Reason" = 'father became king'