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The top level subroutine XYaupd provides the reverse communication
interface to IRAM or IRLM.
The user directly calls subroutine XYaupd in one of the
reverse communication modes to compute the eigenvalues
of the linear operator defined by required for the
computational mode selected by the user.
Every time an operation involving and/or is needed,
prompts the user to provide the action of and/or on a vector
and then re-enter XYaupd. This is the reverse communication interface.
During the initial call to subroutine XYaupd, error checking is performed
on many of the input arguments. The workspace provided to XYaupd is also
partitioned and various counters and pointers are initialized.