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With many numeric values ( e.g. wavelengths) units may be specified. The specified units will be converted into the ones used internally as long as they belong to the same group of units. If no units are specified they are assumed to be equal to the units in which the parameter is expressed inter- nally. The units allowed may be shown interactively by means of the SHOW UNITS command (see section ). In a value-range (see below) units can only be given after the last value of the range ( e.g. 10 TO 100 ANG).
In many places, usually with the operators = and <>, are allowed not only single values but also value-sets and value-ranges.
Value-sets are strings of values, separated by commas and enclosed in parentheses. Blanks are allowed in value-sets, but not within numeric values; blanks are not delimiters. Within value-sets a comma is equivalent to the logical .OR. when used with the = operator, and equivalent to .AND. when used with <>.
Value-ranges can be specified in two ways:
where value1 and value2 are the limits of the range in the first expression and value4 is the centre and value5 the deviation from the centre in the second ( value2 must be > value1, value3 and value5 must be > 0). In both cases value3 is an optional step value. The blanks before and after the words TO, BY and UNC are mandatory. Meaningless ranges are not allowed, and the user will be told. One can often use the help facility to find what is allowed by entering a question mark.
Value ranges may be part of value sets. Frequently used, complicated, value sets and -ranges may be predefined by a symbol; section .