In 4.11.8 we find "A value beginning with a comparator (<, <=, =, >, >=, <>). 6.4.9 "
Now, in 6.4.9 the = and <> comparators are not defined. ("<", "<=", ">", ">=")

4.11.8 defines special behaviour of the = and the <> operators. Cmp. 6.4.7 and 6.4.8 for the infix operators.