Input/Output expressions
I/O expressions are used to process multiple input or output documents in a single map.
I/O expressions can be used to:
      - Read from and write to multiple data sources.
 - Specify the location from which the data should be read or to which it should be written.
 - Switch to a different representation within a map.
 
These expressions are only allowed in elements that inherit from other structures.
The following examples illustrate some use cases for these expressions.