header are normally set by the program which generates
the file, and may be altered (added or modified) by other programs which manipulate
the file, or inherited by children files.
In particular a sequence of HISTORY keywords keep track of the manipulations done.
It is however possible to edit one header keyword at a time also manually using the
appropriate program. This command can be invoked in two ways :
is the form used to edit an existing keyword. If the keyword is an array of numeric values, one can supply as many values as the original array contains. If the keyword is a character string, one can supply a string as long as the original value. In no case a keyword can be extended
beyond the creation length, or changed of type.
is the form used to add a new keyword. New keywords are always scalar quantities. One must
supply also the appropriate keyword data type.
Note that "duplicatable" keywords (typically HISTORY and COMMENT) are always
interpreted as new keywords. It is not possible to alter the previous values of an existing
duplicatable keyword. It is also not possible to assign as a value a string containing