ece108: add intervals, disjoint sets

docs/1b/ece108.md

@@ -355,6 +355,10 @@ The **universal set** is the set containing everything, and is the complement of
 
 $$\mathcal U=\overline\empty$$
 
+Two sets are **disjoint** if they do not have any elements in common.
+
+$$S\cup T=\empty$$
+
 ### Set operations
 
 A **subset** is inside another that is a **superset**.
@@ -391,3 +395,10 @@ $$
 \overline S=\mathcal U-S
 $$
 
+### Intervals
+
+An interval can be represented as a bounded set.
+
+$$[a,b)=\{x\in\mathcal U|a\leq x\wedge x<b\}$$
+
+$\empty$ is any impossible interval.