The signature of a typing: a digest of the input's columns (name, type and nullability) and of each new column's name and lambda. A typing is current while the signature is the same; a column's name is in it, since a later column may use it.
The signature of a typing: a digest of the input's columns (name, type and nullability) and of each new column's name and lambda. A typing is current while the signature is the same; a column's name is in it, since a later column may use it.