May be bound to a function of two arguments. When 'assigning' to
a map key that is absent the function is called passing the map
and the key being applied.

'assigns' val to the key within map. If the meta data
is a map containing the key :proto the corresponding value
will be used to align the type of val, with rounding or
truncation as necessary. If there is no meta data any existing
value is used to maintain the correct type/precision.

An alias for assign

Returns a map that consists of the rest of the maps conj-ed on to
the first, using assign semantics.  If a key occurs in more than
one map, the mapping from the latter (left-to-right) will be the
mapping in the result.