lemma 0.9.2

A pure, declarative language for business rules.
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**Standard library — `uses lemma units`**

Lemma embeds SI bases, derived compounds (force, pressure, energy, power, frequency, electrical), imperial, area/volume, and information (`bit`/`byte`) in `repo lemma` / `spec units`. Import: `uses lemma units`. Reference types: `units.mass`, `units.duration`, `units.length`, `units.force`. Unit names: **singular only** (`8 hour`). Length uses American `meter`. Durations (`hour`, `day`, `week`) require `units.duration`. No Celsius/Fahrenheit (kelvin only).

```lemma
spec logistics
"""
Physical shipment constraints using SI units from the standard library.
"""

uses lemma units

data package_weight: 12 kilogram
data shift_length:   8 hour
data route_distance: 45 kilometer

rule weight_grams:  package_weight as gram
rule shift_hours:   shift_length as hour
rule distance_km:   route_distance as kilometer

rule is_heavy:      package_weight > 20 kilogram
rule is_long_shift: shift_length >= 8 hour
```

Prefer `units.mass`, `units.duration`, `units.length` over redefining units. Convert in family: `as <unit>`. Strip unit: `amount as eur as number`. Cross-family relabel: `5 eur as kg` -> `5 kg`.

**Ranges — half-open intervals**

Ranges: lower bound inclusive, upper bound exclusive (`lo...hi`). Test with `in`. Width: `(lo...hi) as <unit> as number` (or `(lo...hi) as <unit>`). Bare `as number` on date/measure ranges fails. Typedefs: `number range`, `date range`, `measure range`, `ratio range`. Month/year intervals: `uses lemma units` and inline literals (`18 year...67 year`) or `units.calendar range`.

```lemma
spec eligibility
uses lemma units

data employee_age:       42 year
data performance_score:  75
data package_weight:     45 kilogram
data hire_date:          2024-01-15
data review_date:        2024-06-30
data discount_rate:      15%

data eligible_band: units.calendar range
  -> suggest 18 year...67 year

data score_band: number range
  -> suggest 0...100

rule is_working_age: employee_age in eligible_band

rule is_top_score: performance_score in 90...100

rule is_heavy: package_weight in 30 kilogram...80 kilogram

rule in_discount_band: discount_rate in 0%...50%

rule in_q2: hire_date in 2024-04-01...2024-07-01

rule review_days: (hire_date...review_date) as day

rule span_years: (1990-05-20...2024-06-15) as year
```

Upper bound exclusive: `67 year` is NOT inside `18 year...67 year` (returns false). Declare range slots on `data` for reuse, inline `value in lo...hi` for one-off.

Range over custom measure type (no SI import needed):

```lemma
spec freight

data weight: measure
  -> unit gram 1
  -> unit kilogram 1000

data load_band: weight range
  -> suggest 30 kilogram...80 kilogram

rule inside_band: 45 kilogram in load_band

rule band_width: (30 kilogram...80 kilogram) as kilogram
```

**Derived measures — compound units**

Build compound units with `/`, `*`, `^`. Name derived unit, then give compound expression. Prior measure types must declare referenced base units (`eur`, `hour`, `employee`, `results`). Import `uses lemma units` if using time (`eur/hour`).

```lemma
spec contractor
uses lemma units

data money: measure
  -> unit eur 1.00

data headcount: measure
  -> unit employee 1

data outcome: measure
  -> unit results 1

data wage_rate: measure
  -> unit eur_per_second eur/second
  -> unit eur_per_hour eur/hour

data productivity: measure
  -> unit result_per_employee results/eur/hour/employee

data premium_per_head: measure
  -> unit eur_hour_per_employee eur_per_hour/employee

data time_worked: 120 hour
data wage: wage_rate -> suggest 85 eur_per_hour
data yield_rate: productivity -> suggest 3 result_per_employee

rule total: wage * time_worked

rule is_high_yield: yield_rate >= 2 result_per_employee
```

Layer compound units: `eur_per_hour` builds on `eur` and `hour`. Dimensional checks run at plan time.

**Date predicates relative to `now`**

`now` is evaluation/effective instant. Import `uses lemma units` for duration windows.

| Form | Meaning |
|------|---------|
| `date in past` / `in future` | Before / after `now` |
| `date in past N day` / `in future N day` | In last / next N duration units |
| `past N day` / `future N day` | Relative date-range window |
| `date in calendar year\|month\|week` | Current calendar period |
| `date in past\|future calendar year\|month\|week` | Adjacent calendar period |
| `date not in calendar year\|month\|week` | Not current calendar period |

```lemma
spec recency
uses lemma units
data event_date: date
rule recent: event_date in past 7 day
rule this_year: event_date in calendar year
```