///////////
# get-env #
///////////

## outputs value of existing env variable ##
//set-env var test-val
~> get-env var
▶ test-val
~> put $E:var
▶ test-val

## throws if env variable doesn't exist ##
//unset-env var
~> get-env var
Exception: non-existent environment variable
  [tty]:1:1-11: get-env var

///////////
# has-env #
///////////

## exists ##
//set-env var test-val
~> has-env var
▶ $true

## doesn't exist ##
//unset-env var
~> has-env var
▶ $false

///////////
# set-env #
///////////
~> set-env var test-val
~> echo $E:var
test-val