the column in Update

From the 7.4 docs:
 A column can be referenced in the form

correlation.columnname

correlation is the name of a table (possibly qualified with a schema name), or
an alias for a table defined by means of a FROM clause, or one of the key
words NEW or OLD. (NEW and OLD can only appear in rewrite rules, while other
correlation names can be used in any SQL statement.) The correlation name and
separating dot may be omitted if the column name is unique across all the
tables being used in the current query. (See also Chapter 7.)

So then why does this not work:
Update tablename set tablename.columnName = 'somedata' where .....

John