1.less

// Usage:

a {
   .text-transform(upper);
}

b {
   .text-transform(none);
}

c {
   .text-transform(foobar);
}

// etc.

// Impl:
.text-transform(@type: none) {
    .-(type);
    .-(upper)   {text-transform: uppercase}
    .-(lower)   {text-transform: lowercase}
    .-(better)  {text-transform: betterhat}
    // etc.
    .-(...) when (default()) {text-transform: @type}
}

2.less

// Usage:

a {
   .text-transform(upper);
}

b {
   .text-transform(none);
}

c {
   .text-transform(foobar);
}

// etc.

// Impl:
.text-transform(@type: none) {
    .-(type);
    .-(...)     {@t: @type}
    .-(upper)   {@t: uppercase}
    .-(lower)   {@t: lowercase}
    .-(better)  {@t: betterhat}
    // etc.
    text-transform: @t;
}

3.less

// Usage:

a {
   .text-transform(upper);
}

b {
   .text-transform(none);
}

c {
   .text-transform(foobar);
}

// etc.

// Impl:
.text-transform(@type) when (default()) {text-transform: @type}
.text-transform()       {text-transform: none}
.text-transform(upper)  {text-transform: uppercase}
.text-transform(lower)  {text-transform: lowercase}
.text-transform(better) {text-transform: betterhat}
 // etc.