NGRAM TEXT index CONTAINS BEFORE search [CR #642744]

An NGRAM TEXT index search with a CONTAINS… BEFORE clause may fail with the error:

Could not execute statement.
Syntax error -  CONTAINS Near/Before first arguments
must be single term.
-- (df_Heap.cxx 6876)
SQLCODE=-1005028, ODBC 3 State="HY000"

The same query with a CONTAINS… NEAR clause returns a result without error.