Cluster is an assembler (initially for x86 and x86-64) with a
difference.

People seem to be very opinionated about syntax, and that is true for
the input syntax to an assembler as well.

To avoid the issue of defining a syntax, the input to Cluster is a
list of standard objects (i.e., instances of the class
STANDARD-OBJECT), as opposed to a character file or S-expressions.

When used as a backend for compilers, it is more convenient to submit
code to the assembler in this form than to produce (say) S-expressions
that then need to be parsed.