.QUERY EXISTANCE OF FILE: PIC001
[Enter name of file which you want to know if it
exists or not. The file may be a non-SPIDER file,
a SPIDER image file, an image in a SPIDER iamge stack,
an inline SPIDER "file", or an iamge in an inline SPIDER
image stack "file" . If the file exists and a register
is supplied on the operation line, the register's
contents will be set to 1.0. If the file does not
exist the register's contents will be set to 0.0.
If the file is already opened and a second register
is supplied on the operation line, the second register's
contents will be set to 1.0. If the file is not open
the register's contents will be set to 0.0.]