An expression combines field names, constants, and operators
in a calculation that returns a single value. You can use an expression
in a variety of commands to assign a value to a temporary field
or Dialogue Manager amper variable, or use it in screening. You
can combine simpler ones to build increasingly complex expressions.
When you write an expression, you can specify the operation yourself,
or you can use one of the many supplied functions that perform specific
calculations or data manipulation. These functions operate on one
or more arguments, and return a single value as a result. To use
a function, you simply call it. For details about functions, see
the Using Functions manual.