@no-true-branches describe-error :=
  "no tests in condition: block were true"

False and: _ := False
True and: (b: Block) := b call

True or: _ := True
False or: (b: Block) := b call

macro (a && b) `(~a and: { ~b })
macro (a || b) `(~a or: { ~b })

True not := False
False not := True

if: True then: (a: Block) else: Block :=
  a call

if: False then: Block else: (b: Block) :=
  b call

unless: True do: Block := @ok
unless: False do: (action: Block) :=
  { action in-context call
    @ok
  } call

macro (e unless: b) `(unless: ~b do: { ~e })

when: False do: Block := @ok
when: True do: (action: Block) :=
  { action in-context call
    @ok
  } call

macro (e when: b) `(when: ~b do: { ~e })

while: (test: Block) do: (action: Block) :=
  { cc a |
    { (cc yield: @ok) unless: test call
      a call
    } repeat
  } call/cc: action in-context

(action: Block) while: (test: Block) :=
  { action in-context call
    while: test do: action
  } call

until: (test: Block) do: (action: Block) :=
  { cc a |
    { (cc yield: @ok) when: test call
      a call
    } repeat
  } call/cc: action in-context

(action: Block) until: (test: Block) :=
  { action in-context call
    until: test do: action
  } call

otherwise := True

macro (condition: (bs: Block))
  bs contents reduce-right:
    { `(~test -> ~branch) x |
      `(if: ~test then: { ~branch } else: { ~x })
    } with: `(error: @no-true-branches)