Omit '0:' if only one cause

README.md

@@ -57,7 +57,7 @@ anyhow = "=1.0.0-alpha.1"
   Error: failed to read instrs from ./path/to/instrs.jsox
 
   Caused by:
-      0: No such file or directory (os error 2)
+      No such file or directory (os error 2)
   ```
 
 - Downcasting is supported and can be by value, by shared reference, or by

src/context.rs

@@ -45,7 +45,7 @@ use std::backtrace::Backtrace;
 /// Error: failed to read instrs from ./path/to/instrs.jsox
 ///
 /// caused by:
-///     0: No such file or directory (os error 2)
+///     No such file or directory (os error 2)
 /// ```
 pub trait Context<T, E> {
     /// Wrap the error value with additional context.

src/error.rs

@@ -316,10 +316,13 @@ impl Debug for Error {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         writeln!(f, "{}", self.inner.error())?;
 
-        let mut chain = self.chain().skip(1).enumerate();
+        let mut chain = self.chain().skip(1).enumerate().peekable();
         if let Some((n, error)) = chain.next() {
-            writeln!(f, "\nCaused by:")?;
+            write!(f, "\nCaused by:\n    ")?;
-            writeln!(f, "    {}: {}", n, error)?;
+            if chain.peek().is_some() {
+                write!(f, "{}: ", n)?;
+            }
+            writeln!(f, "{}", error)?;
             for (n, error) in chain {
                 writeln!(f, "    {}: {}", n, error)?;
             }

src/lib.rs

@@ -81,7 +81,7 @@
 //!   Error: failed to read instrs from ./path/to/instrs.jsox
 //!
 //!   Caused by:
-//!       0: No such file or directory (os error 2)
+//!       No such file or directory (os error 2)
 //!   ```
 //!
 //! - Downcasting is supported and can be by value, by shared reference, or by