The mapping Dictionary may also contain "write:" Definitions which define values to be assigned to Trace Header fields as the binary is being written out. These are a little different from the mapping Definitions and are needed to provide some of the built-in functions that are only meaningful in the sequence of writing traces. They take on the form:

write:out.field= expression
    

• "out" can be the format name (eg. "usp" or "segy"), alias names (eg. "out" or "vel"), or "*".
  
  
• "field" can be any Trace Header name in the output dataset.
  
  
• "expression" is an algebraic expression. It may contain constants, field names from the input format, operators, built-in functions, and parentheses.

For example, segy header word ReelTrcNum can be assigned new values on output.

write:segy.ReelTrcNum= 1 + reeltrc()
    

This example will assign sequence numbers to "ReelTrcNum" using the built-in mapping function, "reeltrc()".