Each transformation is represented as a Perl array. The first element is a
command that defines the transformation. For example ["noop"] is a no-op
transformation. The remaining fields of the array are arguments. For example,
["echo", "Hello,", "world."] is an echo transformation that prints
Hello, world.. The arguments need not be strings, for example
block is a list of further transformations:
["block",
["echo", "Mary had a little lamb,"],
["echo", "whose fleece was white as snow."],
["echo", "And everywhere that Mary went,"],
["echo", "the lamb was sure to go."],
]
Other transformations take other types of arguments (e.g., aggregate takes a hash).
The command must be one of the 20 basic transformations described in this
document, or a function defined in the %functions hash.