Associates a file name template or any other
string with a string (symbolic) variable.
Reads input from current procedure or terminal.
'FR L' differs from 'FR' which
receives input from the calling procedure or input stream.
The string variable is available only inside the
current SPIDER procedure.
.VARIABLE NAME & ASSOCIATED VALUE: [label]value
[Enter string variable name (surrounded by 's) and
the associated alphanumeric string.]
This operation is useful when writing long procedures as
a string variable can be initialized to contain a certain
value at the top of the procedure then the variable
e.g. [raw_file] can be used throughout the procedure. If the
string's value later changes then it only has to be altered at
one location in the procedure.
When procedure executes, operation sequence is :