i did actually pattern matching holy crap

src/main.rs

@@ -1,6 +1,6 @@
 use core::fmt;
 use std::{
-    collections::HashSet,
+    collections::{HashMap, HashSet},
     env,
     io::{self, Read},
     iter,
@@ -436,6 +436,23 @@ impl Rule {
             rhs: to.clone(),
         }
     }
+
+    #[must_use]
+    fn concrete_with_matches(&self, matches: &Matches<'_>) -> Rule {
+        match self {
+            Rule::Forall { vars, .. } => {
+                let mut out = self.clone();
+
+                for var in vars {
+                    let expr = matches.get(var).copied().cloned().unwrap_or_else(Sexp::nil);
+                    out = out.concrify(&expr);
+                }
+
+                out
+            }
+            Rule::Concrete { .. } => self.clone(),
+        }
+    }
 }
 
 impl fmt::Display for Rule {
@@ -473,45 +490,76 @@ fn could_match(vars: Option<&[Symbol]>, lhs: &Sexp, expr: &Sexp) -> bool {
     }
 }
 
-// type Matches<'src> = HashMap<Symbol, HashSet<&'src Sexp>>;
+type Matches<'src> = HashMap<Symbol, &'src Sexp>;
 
-// TODO: there. can. be. at most. one. match.
+// DONE?: there. can. be. at most. one. match.
 // i'm happy that this is faster, but it might be worth going back to the per-variable thing
-fn matches<'src>(vars: &HashSet<Symbol>, lhs: &Sexp, expr: &'src Sexp) -> HashSet<&'src Sexp> {
+fn matches<'src>(vars: &[Symbol], lhs: &Sexp, expr: &'src Sexp) -> Option<Matches<'src>> {
     match (lhs, expr) {
         (Sexp::Atom(a), Sexp::Atom(b)) => {
-            if a == b || vars.contains(a) {
-                let mut out = HashSet::with_capacity(1);
-                out.insert(expr);
-                out
+            if a == b {
+                Some(HashMap::with_capacity(0))
+            } else if vars.contains(a) {
+                let mut out = HashMap::with_capacity(1);
+                out.insert(*a, expr);
+                Some(out)
             } else {
-                HashSet::with_capacity(0)
+                None
             }
         }
         (Sexp::Atom(a), Sexp::List(_)) => {
             if vars.contains(a) {
-                let mut out = HashSet::with_capacity(1);
-                out.insert(expr);
-                out
+                let mut out = HashMap::with_capacity(1);
+                out.insert(*a, expr);
+                Some(out)
             } else {
-                HashSet::with_capacity(0)
+                None
             }
         }
-        (Sexp::List(_), Sexp::Atom(_)) => HashSet::with_capacity(0),
+        (Sexp::List(_), Sexp::Atom(_)) => None,
         (Sexp::List(xs), Sexp::List(ys)) => {
             if xs.len() == ys.len() {
                 xs.iter()
                     .zip(ys)
                     .map(|(lhs, expr)| matches(vars, lhs, expr))
-                    .reduce(|a, b| a.union(&b).copied().collect())
-                    .unwrap_or(HashSet::with_capacity(0))
+                    .reduce(|a, b| match (a, b) {
+                        (None, _) => None,
+                        (_, None) => None,
+                        (Some(a), Some(b)) => merge_matches(vars, a, b),
+                    })
+                    .unwrap_or(None)
             } else {
-                HashSet::with_capacity(0)
+                None
             }
         }
     }
 }
 
+fn merge_matches<'src>(
+    vars: &[Symbol],
+    a: Matches<'src>,
+    b: Matches<'src>,
+) -> Option<Matches<'src>> {
+    let mut out = HashMap::with_capacity(vars.len());
+
+    for var in vars {
+        match (a.get(var), b.get(var)) {
+            (None, None) => {}
+            (Some(m), None) | (None, Some(m)) => {
+                out.insert(*var, *m);
+            }
+            (Some(a), Some(b)) => {
+                if a != b {
+                    return None;
+                }
+                out.insert(*var, *a);
+            }
+        }
+    }
+
+    Some(out)
+}
+
 // fn absorb_matches<'src>(from: Matches<'src>, into: &mut Matches<'src>) {
 //     for (var, matches) in from {
 //         let insert_into = into.entry(var).or_default();
@@ -527,19 +575,9 @@ fn rw(rule: &Rule, sexp: &Sexp) -> Sexp {
     }
 
     match rule {
-        rule @ Rule::Forall { vars, lhs, .. } => {
-            if could_match(Some(vars), lhs, sexp) {
-                // let targets = sexp.concretion_targets();
-                let targets = matches(&vars.iter().copied().collect(), lhs, sexp);
-                let mut out = sexp.clone();
-                for rule in rule.concretions(&targets) {
-                    let rwed = rw(&rule, &out);
-                    if rwed != out {
-                        out = rwed;
-                        break;
-                    }
-                }
-                out
+        Rule::Forall { vars, lhs, .. } => {
+            if let Some(matc) = matches(vars, lhs, sexp) {
+                rule.concrete_with_matches(&matc).rw(sexp)
             } else {
                 match sexp {
                     Sexp::Atom(_) => sexp.clone(),