Concepts / Language Features
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Quoting

new record syntax
Agents
Vars
state identity lifetime
immutability / side effects
type hints
Pre and Post conditions of functions

Particular Functions
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fnil - creating_a_function
juxt - creating_a_function
constantly - creating_a_function
flatten

frequencies
reductions
keep
keep-indexed
map-indexed
partition-all
partition-by
repeatedly