The function_name procedure must have been previously defined using CREATE FUNCTION and must be defined to accept the correct number of arguments (either one or two) of the indicated types.
"""
This should also include a comment about "in the same order"
"...to accept the correct number of arguments (either one or two and in left to right order) of the indicated types."
Apparently the system does not attempt to find (2 arg) functions with the correct number and types of arguments but in the reverse order.